pwm: stm32: Search an appropriate duty_cycle if period cannot be modified

pwm: stm32: Search an appropriate duty_cycle if period cannot be modified

If another channel is already enabled period must not be modified. If
the requested period is smaller than this unchangable period the driver
is still supposed to search a duty_cycle according to the usual rounding
rules.

So don't set the duty_cycle to 0 but continue to determine an
appropriate value for ccr.

Fixes: deaba9cff809 ("pwm: stm32: Implementation of the waveform callbacks")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/f0c50df31daa3d6069bfa8d7fb3e71fae241b026.1743844730.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>