drivers: fuel_gauge: ltc2959: Fix build warnings from declarations

[ttmut]

C standards prior to C23 do not allow variable declarations immediately after labels. Wrap such declarations in blocks to eliminate build warnings. Should hopefully fix the CI failure occurring in main https://github.com/zephyrproject-rtos/zephyr/actions/runs/18721827138/job/53395989790

Tested locally on native_sim:

$ west build -p -b native_sim/native tests/drivers/fuel_gauge/ltc2959 -T drivers.fuel_gauge.ltc2959 --
-DZEPHYR_TOOLCHAIN_VARIANT=llvm

/home/tahsin/zephyr/zephyr/drivers/fuel_gauge/ltc2959/ltc2959.c:465:3: warning: label followed by a declaration is a C23 extension [-Wc23-extensions]
  465 |                 uint8_t raw_st;
      |                 ^
/home/tahsin/zephyr/zephyr/drivers/fuel_gauge/ltc2959/ltc2959.c:478:3: warning: label followed by a declaration is a C23 extension [-Wc23-extensions]
  478 |                 uint16_t raw_voltage;
      |                 ^
/home/tahsin/zephyr/zephyr/drivers/fuel_gauge/ltc2959/ltc2959.c:495:3: warning: label followed by a declaration is a C23 extension [-Wc23-extensions]
  495 |                 uint16_t raw_current;
      |                 ^
/home/tahsin/zephyr/zephyr/drivers/fuel_gauge/ltc2959/ltc2959.c:511:3: warning: label followed by a declaration is a C23 extension [-Wc23-extensions]
  511 |                 uint16_t raw_temp;
      |                 ^
/home/tahsin/zephyr/zephyr/drivers/fuel_gauge/ltc2959/ltc2959.c:530:3: warning: label followed by a declaration is a C23 extension [-Wc23-extensions]
  530 |                 uint32_t acr;
      |                 ^
/home/tahsin/zephyr/zephyr/drivers/fuel_gauge/ltc2959/ltc2959.c:640:3: warning: label followed by a declaration is a C23 extension [-Wc23-extensions]
  640 |                 uint32_t counts = ltc2959_uah_to_counts(val.remaining_capacity, cfg);
      |                 ^
6 warnings generated.
[110/112] Linking C executable zephyr/zephyr.elf
Generating files from /home/tahsin/zephyr/zephyr/build/zephyr/zephyr.elf for board: native_sim
[112/112] Running utility command for native_runner_executable

Warnings go away after applying the patch:

-- west build: building application
[1/112] Preparing syscall dependency handling

[3/112] Generating include/generated/zephyr/version.h
-- Zephyr version: 4.2.99 (/home/tahsin/zephyr/zephyr), build: v4.2.0-6706-g4842828358b5
[110/112] Linking C executable zephyr/zephyr.elf
Generating files from /home/tahsin/zephyr/zephyr/build/zephyr/zephyr.elf for board: native_sim
[112/112] Running utility command for native_runner_executable

[ttmut]

#90356 (comment)

FYI @jhedberg

[ttmut]

Force-pushed to fix compliance failures regarding signed-off entries.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[DBS06]

Interesting issue

[LostinTimeandspaceYT]

@ttmut Thank you for patching this fix in!