drivers/rtc/rtc-ds1685.c: fix sparse warnings

Fix two minor sparse warnings:

  CHECK   drivers/rtc/rtc-ds1685.c
  drivers/rtc/rtc-ds1685.c:2178:1: warning: function 'ds1685_rtc_poweroff' with external linkage has definition
  drivers/rtc/rtc-ds1685.c:802:23: warning: Using plain integer as NULL pointer

Fixes: aaaf5fbf ("rtc: add driver for DS1685 family of real time clocks")
Signed-off-by: Joshua Kinard <kumba@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>