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

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: aaaf5fbf56f1 ("rtc: add driver for DS1685 family of real time clocks")
Signed-off-by: Joshua Kinard <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>