drivers/rtc/rtc-m41t93.c: don't let get_time() reset M41T93_FLAG_OF

If the rtc reports the time might be invalid due to oscillator failure,
M41T93_FLAG_OF flag must not be reset by get_time() as the read operation
doesn't make the time valid.

Without this patch, only the first get_time() reported an invalid time,
the second get_time() reported a valid time althought the reported time is
probably wrong due to oscillator failure.

Instead of resetting in get_time(), with this patch M41T93_FLAG_OF is
reset in set_time() when a valid time is to be written.

Signed-off-by: Nikolaus Voss <n.voss@weinmann.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>