shinelanx-modbus-gw: increase busy wait

There seem to be inverters which are slower. As a workaround for the
workaround increase the time to 10s. The proper solution would be to
wait until the port has been opened.

Fixes #3.

apps/shinelanx-modbus-gw/main.c

@@ -152,7 +152,7 @@ int main(int argc, FAR char *argv[])
 #endif
 
 	/* XXX wait until ttyACM is available */
-	sleep(1);
+	sleep(10);
 
 	mbusd_main(nitems(mbusd_main_args), mbusd_main_args);