rtc: test before subtraction on unsigned

new_alarm is unsigned so test before the subtraction.

[akpm@linux-foundation.org: time-wrapping fix]
Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: David Brownell <david-b@pacbell.net>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>