drivers/rtc/rtc-ds1742.c: report to RTC core if retrieved time is invalid

Let RTC core decide if the retrieved time is invalid, instead of
processing errors in the driver.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Cc: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>