rtc: rs5c372: fix buffer size

Match the buffer size to the amount of initialized values.  Before, it was
one too big and thus destroyed the neighbouring register causing the clock
to run at false speeds.

Reported-by: Andre van Rooyen <a.v.rooyen@sercom.nl>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>