make ds1511_rtc_{read,set}_time() static

Make the needlessly global ds1511_rtc_{read,set}_time() static.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
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>