net: broadcom/bcmsysport: Fix signedness in bcm_sysport_probe()

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

Fixes: 80105bef ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>