rtc: omap: fix error path when pinctrl_register fails

If pinctrl_register() fails probe will return with an error without locking
the RTC and disabling pm_runtime.
Set ret and jump to err instead.

Fixes: 97ea1906 ("rtc: omap: Support ext_wakeup configuration")
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>