pwm: iqs620a: Replace one remaining instance of regmap_update_bits()

The call to regmap_update_bits() which was responsible for clearing
the PWM output enable register bit was recently dropped in favor of
a call to regmap_clear_bits(), thereby simplifying the code.

Similarly, the call to regmap_update_bits() which sets the same bit
can be simplified with a call to regmap_set_bits().

Signed-off-by: Jeff LaBundy <jeff@labundy.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>