e1000: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of just letting the code
fall through to the next case.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/e1000/e1000_hw.c

@@ -1183,6 +1183,7 @@ static s32 e1000_copper_link_igp_setup(struct e1000_hw *hw)
 			break;
 		case e1000_ms_auto:
 			phy_data &= ~CR_1000T_MS_ENABLE;
+			break;
 		default:
 			break;
 		}