Partially revert back STM32.PWM changes.
Partially revert back STM32.PWM changes.

Attach_PWM_Channel does not enable the channel automatically, as documented
in the post-condition.
An explicit call to Enable_PWM_Channel needs to be performed instead.

Fabien-Chouteau/Ada_Drivers_Library,AdaCore/Ada_Drivers_Library,Fabien-Chouteau/Ada_Drivers_Library,simonjwright/Ada_Drivers_Library,AdaCore/Ada_Drivers_Library,lambourg/Ada_Drivers_Library,ellamosi/Ada_BMP_Library,simonjwright/Ada_Drivers_Library