igb: do not allow phy sw reset code to make calls to null pointers

In the case of fiber and serdes adapters we were seeing issues with ethtool
-t causing kernel panics due to null function pointers.  To prevent this we
need to exit out of the phy reset code in the event that we do not have a
valid phy.
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/igb/e1000_phy.c

@@ -1565,9 +1565,12 @@ out:
  **/
 s32 igb_phy_sw_reset(struct e1000_hw *hw)
 {
-	s32 ret_val;
+	s32 ret_val = 0;
 	u16 phy_ctrl;
 
+	if (!(hw->phy.ops.read_reg))
+		goto out;
+
 	ret_val = hw->phy.ops.read_reg(hw, PHY_CONTROL, &phy_ctrl);
 	if (ret_val)
 		goto out;