drivers: pwm: stm32: fix PWM channel disabling
drivers: pwm: stm32: fix PWM channel disabling

Move period and pulse computation to right before
the channel enable code.

That fixes the inability to disable the channel by
providing the period of 0.

Signed-off-by: Georgij Cernysiov <bd22e7942bc8beaebb3a0cc77ee92e4585e26366@gmail.com>

zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr