rockchip: pwm: add mask for config setting

Use mask to clear old setting before direct set the new config,
or else there it will mess up the config when it's not the same
with default value.

Fixes: 38510598 rockchip: Setup default PWM flags
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>