rtc-ds1307 becomes new-style i2c driver

Convert the rtc-ds1307 driver into a "new style" driver.

Also improve probe() checks:  be more correct about switching out of
AM/PM mode, and issue a (debug) diagnostic when failing due to bogus
register values.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Andrew Victor <andrew@sanpeople.com>
Cc: Bill Gatliff <bgat@billgatliff.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Kumar Gala <galak@gate.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>