pwm: npcx: only reconfigure PWM if necessary

Currently pwm_npcx_pin_set() disables and reconfigures the PWM
controller every time its called, causing the PWM line to pulse even if
only the duty cycle is changed.

Modify the function so that controller is only disabled if any of the
configuration has to be changed, only set the new DCR otherwise.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>