smsc75xx: SMSC LAN75xx USB gigabit ethernet adapter driver

This patch adds a driver for SMSC's LAN7500 family of USB 2.0
to gigabit ethernet adapters.  It's loosely based on the smsc95xx
driver but the device registers for LAN7500 are completely different.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>