smsc75xx: fix phy interrupt acknowledge

smsc75xx phy interrupt acknowledge needs an mdio_write to clear
PHY_INT_SRC instead of just a read like in smsc95xx.

Signed-off-by: Stephane Fillod <fillods@users.sf.net>
Signed-off-by: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: David S. Miller <davem@davemloft.net>