net: netsec: Fix signedness bug in netsec_probe()

The "priv->phy_interface" variable is an enum and in this context GCC
will treat it as an unsigned int so the error handling is never
triggered.

Fixes: 533dd11a ("net: socionext: Add Synquacer NetSec driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>