net: phy: don't crash in phy_read/_write_mmd without a PHY driver

The APIs can be used by Ethernet drivers without actually loading a PHY
driver. This may become more widespread in the future with 802.3z
compatible MAC PCS devices being locally driven by the MAC driver when
configuring for a PHY mode with in-band negotiation.

Check that drv is not NULL before reading from it.

Signed-off-by: Alex Marginean <[e-mail]>
Signed-off-by: Vladimir Oltean <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/phy/phy-core.c

···
 	if (regnum > (u16)~0 || devad > 32)
 		return -EINVAL;
 
-	if (phydev->drv->read_mmd) {
+	if (phydev->drv && phydev->drv->read_mmd) {
 		val = phydev->drv->read_mmd(phydev, devad, regnum);
 	} else if (phydev->is_c45) {
 		u32 addr = MII_ADDR_C45 | (devad << 16) | (regnum & 0xffff);
···
 	if (regnum > (u16)~0 || devad > 32)
 		return -EINVAL;
 
-	if (phydev->drv->write_mmd) {
+	if (phydev->drv && phydev->drv->write_mmd) {
 		ret = phydev->drv->write_mmd(phydev, devad, regnum, val);
 	} else if (phydev->is_c45) {
 		u32 addr = MII_ADDR_C45 | (devad << 16) | (regnum & 0xffff);