drivers: pwm: pwm_mcux_sctimer: duty cycle fix

This is a fix for a driver bug that assumes a
user will want a new pwm channel for a new
pwm signal if they decide to change the
period length of the pwm. In some cases,
this creates a noticable change of
duty cycle accuracy.

Signed-off-by: Emilio Benavente <emilio.benavente@nxp.com>