drivers/rtc/rtc-r9701.c: reset registers if invalid values are detected

hwclock refuses to set date/time if RTC registers contain invalid
values.  Check the date/time register values at probe time and
initialize them to make hwclock happy.

Signed-off-by: Andreas Dumberger <andreas.dumberger@tqs.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>