dm: i2c: Implement driver model support in the i2c command

The concept of a 'current bus' is now implemented in the command line
rather than in the uclass. Also the address length does not need to
be specified with each command - really we should consider dropping
this from most commands but it works OK for now.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>