pwm: imx27: Simplify helper function to enable and disable clocks

pwm_imx27_clk_prepare_enable() took a pointer to a struct pwm_chip just
to convert it to a struct pwm_imx27_chip pointer while all callers
already have the latter. Ditto for pwm_imx27_clk_disable_unprepare().

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