pwm: clk: Use the devm_clk_get_prepared() helper function

Use the devm_clk_get_prepared() helper function instead of hand-writing it.
It saves some line of codes.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>