pwm: stm32-lp: Add check in case requested period cannot be achieved

pwm: stm32-lp: Add check in case requested period cannot be achieved

LPTimer can use a 32KHz clock for counting. It depends on clock tree
configuration. In such a case, PWM output frequency range is limited.
Although unlikely, nothing prevents user from requesting a PWM frequency
above counting clock (32KHz for instance):
- This causes (prd - 1) = 0xffff to be written in ARR register later in
the apply() routine.
This results in badly configured PWM period (and also duty_cycle).
Add a check to report an error is such a case.

Signed-off-by: Fabrice Gasnier <[email protected]>
Reviewed-by: Uwe Kleine-König <[email protected]>
Signed-off-by: Thierry Reding <[email protected]>