Add the PHY_ID_FMT macro to ensure that the format of the id string used by a
driver to match to its specific phy is consistent between the mdio_bus and the
driver.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>