hso: fix NULL-deref on tty open

Fix NULL-pointer dereference on tty open due to a failure to handle a
missing interrupt-in endpoint when probing modem ports:

	BUG: kernel NULL pointer dereference, address: 0000000000000006
	...
	RIP: 0010:tiocmget_submit_urb+0x1c/0xe0 [hso]
	...
	Call Trace:
	hso_start_serial_device+0xdc/0x140 [hso]
	hso_serial_open+0x118/0x1b0 [hso]
	tty_open+0xf1/0x490

Fixes: 542f548 ("tty: Modem functions for the HSO driver")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: jhovold

drivers/net/usb/hso.c

@@ -2620,14 +2620,18 @@ static struct hso_device *hso_create_bulk_serial_device(
 		 */
 		if (serial->tiocmget) {
 			tiocmget = serial->tiocmget;
+			tiocmget->endp = hso_get_ep(interface,
+						    USB_ENDPOINT_XFER_INT,
+						    USB_DIR_IN);
+			if (!tiocmget->endp) {
+				dev_err(&interface->dev, "Failed to find INT IN ep\n");
+				goto exit;
+			}
+
 			tiocmget->urb = usb_alloc_urb(0, GFP_KERNEL);
 			if (tiocmget->urb) {
 				mutex_init(&tiocmget->mutex);
 				init_waitqueue_head(&tiocmget->waitq);
-				tiocmget->endp = hso_get_ep(
-					interface,
-					USB_ENDPOINT_XFER_INT,
-					USB_DIR_IN);
 			} else
 				hso_free_tiomget(serial);
 		}