pwm: imx27: Only enable the clocks once in .get_state()

Currently the function pwm_imx27_get_state() of enables the clocks once
unconditionally at the start, a second time if the PWM is enabled and
disables unconditionally at the end.

Simplify that to enable once at the start and disable conditionally at
the end.

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