smsc9420: handle magic field of ethtool_eeprom

ethtool.h says the driver should set the magic field in get_eeprom and
verify it in set_eeprom.  This patch adds this functionality using an
arbitary driver-specific magic value constant (0x9420).

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