drivers/rtc/rtc-max6902.c: use register defines instead of magic numbers

Current code has defines for registers, use the defines rather than
magic numbers.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Cc: Raphael Assenat <raph@raphnet.net>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>