cmd/swarm: Exit if --ethapi is set

The previous attempt to use --ethapi as a fallback if --ens-api is not
set does not work because --ens-api has a default value, and also
setting --ens-api to "" is the suggested way to disable ENS lookups.

Signed-off-by: Lewis Marshall <lewis@lmars.net>

cmd/swarm/run_test.go

@@ -194,7 +194,7 @@ func newTestNode(t *testing.T, dir string) *testNode {
 		"--nodiscover",
 		"--datadir", dir,
 		"--ipcpath", conf.IPCPath,
-		"--ethapi", "",
+		"--ens-api", "",
 		"--bzzaccount", account.Address.String(),
 		"--bzznetworkid", "321",
 		"--bzzport", httpPort,