phy: increase size of MII_BUS_ID_SIZE and bus_id

Some bus names are pretty long and do not fit into
17 chars. Increase therefore MII_BUS_ID_SIZE and
phy_fixup.bus_id to larger number. Now mii_bus.id
can host larger name.

Signed-off-by: Volodymyr Bendiuga <volodymyr.bendiuga@gmail.com>
Signed-off-by: Magnus Öberg <magnus.oberg@westermo.se>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>