pwm: zx: Add missing cleanup in error path

zx_pwm_probe() called clk_prepare_enable() before; this must be undone
in the error path.

Fixes: 4836193c ("pwm: Add ZTE ZX PWM device driver")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>