phylib: phy_mii_ioctl() fixes

Make the SIOCGMIIPHY case fall through properly (it is supposed
to not only return the ID of the default PHY but also to read from
that PHY), and make phy_mii_ioctl() return the same error code as
generic_mii_ioctl() in case of an unsupported operation.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>