rtc: fix handling of missing tm_year data when reading alarms

When fixing up invalid years rtc_read_alarm() was calling rtc_valid_tm()
as a boolean but rtc_valid_tm() returns zero on success or a negative
number if the time is not valid so the test was inverted.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>