i2c: Let new-style drivers implement attach_adapter

While it isn't the way the standard device binding model works, it is
OK for new-style drivers to implement attach_adapter. It may help
convert the renaming legacy drivers to new style drivers faster.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: David Brownell <dbrownell@users.sourceforge.net>