pwm: i.MX: remove unnecessary if in pwm_[en|dis]able

The PWM core makes sure that pwm_enable/disable are called only
once. Still keep the enabled state since we will need it in
pwm_config.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>