Adds a phy_mask field to struct mii_bus and uses it.  This field
indicates each phy address to be ignored when probing the mdio bus.

This support is needed for the fs_enet and ibm_emac drivers to be
converted to the generic phy layer among other drivers. Many systems
lock up on probing certain phy addresses or probing doesn't return
0xffff when nothing is found at the address. A new driver I'm
working on also makes use of this mask.
Signed-off-by: NMatt Porter <mporter@kernel.crashing.org>
Signed-off-by: NJeff Garzik <jgarzik@pobox.com>