[ethernet] Fix eth_mc_hash() return status

eth_mc_hash() was missing a return within a switch statement, and so
always falling through to the default case and returning failure.

src/net/ethernet.c

 		ll_addr_bytes[3] = net_addr_bytes[1] & 0x7f;
 		ll_addr_bytes[4] = net_addr_bytes[2];
 		ll_addr_bytes[5] = net_addr_bytes[3];
+		return 0;
 	default:
 		return -ENOTSUP;
 	}