[nrf fromlist] fs: zms: multiple style fixes from previous PR review

This resolves some addressed comments in this PR zephyrproject-rtos#77930
as well as this PR zephyrproject-rtos#80407

Upstream PR #: 80628

Signed-off-by: Riadh Ghaddab <[email protected]>
(cherry picked from commit f8d7586b88a8f80a5dae88caee9d6cdadf67101f)

Author: rghaddab