rtc: ds1307: remove superfluous initialization

ds1307 was kzalloced, so no need to zero members of the struct.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>