pwm: imx: Don't drop the enable bit once set

Changing the duty-cycle should not blindly override (and clear) the
enable (EN) bit if it has already been set. For instance, a PWM
backlight can be enabled and set to a specific intensity using two
operations. The order of these operations should not matter.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>