rtc-ds1307: typo fix found by coverity

Fix a typo turned up by a Coverity check:  referring to the wrong register,
which could cause problems with DS1338 RTCs whose oscillators halted.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>