rtc: disallow update interrupts when time is invalid

Never enable update interrupts when the time set on the rtc is invalid.
In that case, also avoid enabling the emulation because it will fail for
the same reason.

Link: https://lore.kernel.org/r/20191021155631.3342-2-alexandre.belloni@bootlin.com
Link: https://lore.kernel.org/r/CA+ASDXMarBG5C1Kz42B9i_iVZ1=i6GgH9Yja2cdmSueKD_As_g@mail.gmail.com


Reported-by: Jeffy Chen <jeffy.chen@rock-chips.com>
Reported-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>