drivers/rtc/rtc-efi.c: check for invalid data coming back from UEFI

In particular seeing zero in eft->month is problematic, as it results in
-1 (converted to unsigned int, i.e.  yielding 0xffffffff) getting passed
to rtc_year_days(), where the value gets used as an array index
(normally resulting in a crash).  This was observed with the driver
enabled on x86 on some Fujitsu system (with possibly not up to date
firmware, but anyway).

Perhaps efi_read_alarm() should not fail if neither enabled nor pending
are set, but the returned time is invalid?

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reported-by: Raymund Will <rw@suse.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Jingoo Han <jg1.han@samsung.com>
Acked-by: Lee, Chun-Yi <jlee@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>