e1000e: improper return code signage

The e1000_get_cable_length_82577() should return a negative value upon
error.
Signed-off-by: NBruce Allan <bruce.w.allan@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/e1000e/phy.c

@@ -3137,7 +3137,7 @@ s32 e1000_get_cable_length_82577(struct e1000_hw *hw)
 	         I82577_DSTATUS_CABLE_LENGTH_SHIFT;
 
 	if (length == E1000_CABLE_LENGTH_UNDEFINED)
-		ret_val = E1000_ERR_PHY;
+		ret_val = -E1000_ERR_PHY;
 
 	phy->cable_length = length;