These 4 drivers have identical full duplex flow control resolution
functions.  This patch changes them all to use one common function.

The function in question decides whether a device should enable TX and
RX flow control in a standard way (IEEE 802.3-2005 table 28B-3), so this
should also be useful for other drivers.
Signed-off-by: NSteve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>