pwm: jz4740: Use regmap_{set,clear}_bits

Simplify a bit the code by using regmap_set_bits() and
regmap_clear_bits() instead of regmap_update_bits() when possible.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>