smsc75xx: enable mac to detect speed/duplex from phy

This patch sets the automatic speed and duplex detection bits
in MAC_CR to enable the mac to determine its speed automatically
from the phy.

Note this must be done BEFORE the receiver or transmitter is
enabled.

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>