rtc: omap: fix resource leak in registration error path

Make sure to deregister the pin controller in case rtc registration
fails.

Fixes: 57072758 ("rtc: omap: switch to rtc_register_device")
Cc: stable <stable@vger.kernel.org>     # 4.14
Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>