drivers: pwm: mchp_xec: fix PWM control register updates

The pwm_mchp_xec driver doesn't clear the divisor or clock select fields
of the config register value before writing it back.  If the register
was previously written, the new values were being logically OR'd with
the prior values.

Signed-off-by: Jeff Daly <jeffd@silicom-usa.com>