pwm: lpc18xx-sct: Simplify using devm_clk_get_enabled()

With devm_clk_get_enabled() the call to clk_disable_unprepare() can be
dropped from the error path and the remove callback.

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