net: nixge: Fix a signedness bug in nixge_probe()

The "priv->phy_mode" is an enum and in this context GCC will treat it
as an unsigned int so it can never be less than zero.

Fixes: 492caffa ("net: ethernet: nixge: Add support for National Instruments XGE netdev")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>