net: fec: fix ethtool set_pauseparam duplex bug

Setting the pause parameters causes a running network interface to be
restarted.  However, the restart forces the FEC into half-duplex mode,
whether or not the remote end is in half-duplex mode.  Misconfigured
duplex mode is a known source of problems on a link.

Fix this by always preserving the duplex mode on configuration changes.

Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>