drivers: pwm: stm32: reduce pointer level from cfg->timer to timer

This change reduces the level of pointer indirection, which minimizes
repeated dereferencing and helps reduce the overall code size, in the same
way as commit 48e326a for UART.

This also fixes the type complimentary -> complementary.

Signed-off-by: Guillaume Gautier <[email protected]>

Author: gautierg-st