igb: e1000_phy: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 114800 ("Missing break in switch")
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/igb/e1000_phy.c

@@ -659,6 +659,7 @@ s32 igb_copper_link_setup_m88_gen2(struct e1000_hw *hw)
 			phy_data |= M88E1000_PSCR_AUTO_X_1000T;
 			break;
 		}
+		/* fall through */
 	case 0:
 	default:
 		phy_data |= M88E1000_PSCR_AUTO_X_MODE;