rtc-ds1307: SMBus compatibility

Change i2c access functions to SMBus access functions in order to use the
ds1307 with SMBus adapter.

Signed-off-by: Sebastien Barre <sbarre@sdelcc.com>
Acked-by: David Brownell <david-b@pacbell.net>
Tested-by: David Brownell <david-b@pacbell.net>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Acked-by: Jean Delvare <khali@linux-fr.org>
Cc: Rodolfo Giometti <giometti@enneenne.com>
Tested-by: Sebastien Barre <sbarre@sdelcc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>