lte/alt1250: Add FIONBIO ioctl command support

From next change, FIONBIO command will be requested in
usrsock daemon. ALT1250 daemon doesn't use this command,
so only return OK for this command.

  84b8820 serial/pty:Don't assert EBUSY when pty ... driver
  88e871b nuttx/pty: pty FIONBIO return -ENOTTY when ... OK
  37730a1 nuttx/fcntl:pass O_NONBLOCK flag to ioctl
  c7c9148 nuttx/pty: pass read nonblock flag to pipe
  1afdb06 nuttx/pty:pty FIONBIO pass to pipe control

lte/alt1250/alt1250_usockif.c

@@ -257,6 +257,9 @@ int usockif_readreqioctl(int fd, FAR struct usrsock_request_buff_s *buf)
 
   switch (req->cmd)
     {
+      case FIONBIO:
+        rsize = sizeof(int);
+        break;
       case SIOCLTECMD:
         rsize = sizeof(struct lte_ioctl_data_s);
         break;

lte/alt1250/usock_handlers/alt1250_ioctlhdlr.c

@@ -63,6 +63,11 @@ int usockreq_ioctl(FAR struct alt1250_s *dev,
     {
       switch (request->cmd)
         {
+          case FIONBIO:
+            /* ALT1250 doesn't use this command. Only return OK. */
+
+            *usock_result = OK;
+            break;
           case SIOCLTECMD:
             ioctl_subhdlr = usockreq_ioctl_ltecmd;
             break;