rtc: omap: fix potential crash on power off

Do not set the system power-off callback and omap power-off rtc pointer
until we're done setting up our device to avoid leaving stale pointers
around after a late probe error.

Fixes: 97ea1906 ("rtc: omap: Support ext_wakeup configuration")
Cc: stable <stable@vger.kernel.org>     # 4.9
Cc: Marcin Niestroj <m.niestroj@grinn-global.com>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>