rtc: use set_mmss when set_time is not available

Drivers should only need to implement either set_mmss (counter based RTCs)
or set_time (most RTCs).  The RTC subsystem will handle them
appropriately.

Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: David Brownell <david-b@pacbell.net>
Cc: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>