rtc: rtc-ds1216 fixes

Fixes a few issues with the rtc-ds1216 driver

- use rtc_valid_tm

- use platform_driver_probe

- fix init sequence - it was using rtc_unregister_driver where not
  needed.  I also added resource_size and removed an useless pointer
  assignment.

Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Tested-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>