rtc: rtc-nuc900: fix loop timeout test

We should change this post-op to a pre-op because we want the loop to
exit with "timeout" set to zero.

Fixes: 0a89b553 ("nuc900/rtc: change the waiting for device ready implement")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>