pch_gbe: fix if condition in set_settings()

There were no curly braces in this if condition so it always enabled full
duplex.

And ecmd->speed is an unsigned short so it is never equal to -1.  The
effect is that mii_ethtool_sset() fails with -EINVAL and an error is
printed to dmesg.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>