rtc/max6900: use rtc_valid_tm() to check returning tm

Use rtc_valid_tm() to check returning tm for max6900, it can avoid
returning wrong tm value.

Signed-off-by: Wan ZongShun <mcuos.com@gmail.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>
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>