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>

drivers/net/ethernet/broadcom/bcmsysport.c

@@ -2481,7 +2481,7 @@ static int bcm_sysport_probe(struct platform_device *pdev)
 
 	priv->phy_interface = of_get_phy_mode(dn);
 	/* Default to GMII interface mode */
-	if (priv->phy_interface < 0)
+	if ((int)priv->phy_interface < 0)
 		priv->phy_interface = PHY_INTERFACE_MODE_GMII;
 
 	/* In the case of a fixed PHY, the DT node associated