rtc: rtc_update_irq_enable: rework UIE emulation

Now that the core is aware of whether alarms are available, it is possible
to decide whether UIE emulation is required before actually trying to set
the alarm.

This greatly simplifies rtc_update_irq_enable because there is now only one
error value to track and is not relying on the return value of
__rtc_set_alarm anymore.

Tested-by: Łukasz Stelmach <l.stelmach@samsung.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210418000023.995758-3-alexandre.belloni@bootlin.com