rtc: pfc8563: fix uninitialized variable warning

Gcc is unable to prove that alm_pending is always initialized
when it is used, so it prints a harmless warning:

drivers/rtc/rtc-pcf8563.c: In function 'pcf8563_probe':
drivers/rtc/rtc-pcf8563.c:449:5: warning: 'alm_pending' may be used uninitialized in this function [-Wmaybe-uninitialized]

This uses the same conditional expression that is used inside of
the pcf8563_get_alarm_mode() function, to help gcc figure it out
and shut up that warning, and make the ARM defconfigs build again
with no warnings.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: a45d528a ("rtc: pcf8563: clear expired alarm at boot time")
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>