sh: sh-rtc invalid time rework

This patch modifies invalid time handling in the
SuperH RTC driver. Instead of zeroing the returned
value at read-out time we just return an error code
and reset invalid values during probe.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>