[usb-moded] Check return of the dhcp config write

We did not check the return value of the dhcp configuration writing,
which caused mode setting to fail if we did not set up ip forwarding.

Signed-off-by: Philippe De Swert <philippe.deswert@jollamobile.com>

src/usb_moded-network.c

@@ -617,7 +617,7 @@ int usb_network_set_up_dhcpd(struct mode_list_elem *data)
 #endif /*CONNMAN */
   }
   /* ipforward can be NULL here, which is expected and handled in this function */
-  write_udhcpd_conf(ipforward, data);
+  ret = write_udhcpd_conf(ipforward, data);
 
   if(data->nat)
 	ret = set_usb_ip_forward(data, ipforward);