mt7601u: fix kernel crash unplugging the device

The following crash log can occur unplugging the usb dongle since,
after the urb poison in mt7601u_free_tx_queue(), usb_submit_urb() will
always fail resulting in a skb kfree while the skb has been already
queued.

Fix the issue enqueuing the skb only if usb_submit_urb() succeed.

Hardware name: Hewlett-Packard 500-539ng/2B2C, BIOS 80.06 04/01/2015
Workqueue: usb_hub_wq hub_event
RIP: 0010:skb_trim+0x2c/0x30
RSP: 0000:ffffb4c88005bba8 EFLAGS: 00010206
RAX: 000000004ad483ee RBX: ffff9a236625dee0 RCX: 000000000000662f
RDX: 000000000000000c RSI: 0000000000000000 RDI: ffff9a2343179300
RBP: ffff9a2343179300 R08: 0000000000000001 R09: 0000000000000000
R10: ffff9a23748f7840 R11: 0000000000000001 R12: ffff9a236625e4d4
R13: ffff9a236625dee0 R14: 0000000000001080 R15: 0000000000000008
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fd410a34ef8 CR3: 00000001416ee001 CR4: 00000000001706f0
Call Trace:
 mt7601u_tx_status+0x3e/0xa0 [mt7601u]
 mt7601u_dma_cleanup+0xca/0x110 [mt7601u]
 mt7601u_cleanup+0x22/0x30 [mt7601u]
 mt7601u_disconnect+0x22/0x60 [mt7601u]
 usb_unbind_interface+0x8a/0x270
 ? kernfs_find_ns+0x35/0xd0
 __device_release_driver+0x17a/0x230
 device_release_driver+0x24/0x30
 bus_remove_device+0xdb/0x140
 device_del+0x18b/0x430
 ? kobject_put+0x98/0x1d0
 usb_disable_device+0xc6/0x1f0
 usb_disconnect.cold+0x7e/0x20a
 hub_event+0xbf3/0x1870
 process_one_work+0x1b6/0x350
 worker_thread+0x53/0x3e0
 ? process_one_work+0x350/0x350
 kthread+0x11b/0x140
 ? __kthread_bind_mask+0x60/0x60
 ret_from_fork+0x22/0x30

Fixes: 23377c20 ("mt7601u: fix possible memory leak when the device is disconnected")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Acked-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/3b85219f669a63a8ced1f43686de05915a580489.1610919247.git.lorenzo@kernel.org

drivers/net/wireless/mediatek/mt7601u/dma.c

@@ -309,7 +309,6 @@ static int mt7601u_dma_submit_tx(struct mt7601u_dev *dev,
 	}
 
 	e = &q->e[q->end];
-	e->skb = skb;
 	usb_fill_bulk_urb(e->urb, usb_dev, snd_pipe, skb->data, skb->len,
 			  mt7601u_complete_tx, q);
 	ret = usb_submit_urb(e->urb, GFP_ATOMIC);
@@ -327,6 +326,7 @@ static int mt7601u_dma_submit_tx(struct mt7601u_dev *dev,
 
 	q->end = (q->end + 1) % q->entries;
 	q->used++;
+	e->skb = skb;
 
 	if (q->used >= q->entries)
 		ieee80211_stop_queue(dev->hw, skb_get_queue_mapping(skb));