smsc95xx: add phylib support

Generally, each PHY has their own configuration and it can be done
through an external PHY driver.  The smsc95xx driver uses only the
hard-coded internal PHY configuration.

This patch adds phylib support to probe external PHY drivers for
configuring external PHYs.

The MDI-X configuration for the internal PHYs moves from
drivers/net/usb/smsc95xx.c to drivers/net/phy/smsc.c.

Signed-off-by: Andre Edich <andre.edich@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>