mt7601u: do not schedule rx_tasklet when the device has been disconnected

Do not schedule rx_tasklet when the usb dongle is disconnected.
Moreover do not grub rx_lock in mt7601u_kill_rx since usb_poison_urb
can run concurrently with urb completion and we can unlink urbs from rx
ring in any order.
This patch fixes the common kernel warning reported when
the device is removed.

[   24.921354] usb 3-14: USB disconnect, device number 7
[   24.921593] ------------[ cut here ]------------
[   24.921594] RX urb mismatch
[   24.921675] WARNING: CPU: 4 PID: 163 at drivers/net/wireless/mediatek/mt7601u/dma.c:200 mt7601u_complete_rx+0xcb/0xd0 [mt7601u]
[   24.921769] CPU: 4 PID: 163 Comm: kworker/4:2 Tainted: G           OE     4.19.31-041931-generic #201903231635
[   24.921770] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./Z97 Extreme4, BIOS P1.30 05/23/2014
[   24.921782] Workqueue: usb_hub_wq hub_event
[   24.921797] RIP: 0010:mt7601u_complete_rx+0xcb/0xd0 [mt7601u]
[   24.921800] RSP: 0018:ffff9bd9cfd03d08 EFLAGS: 00010086
[   24.921802] RAX: 0000000000000000 RBX: ffff9bd9bf043540 RCX: 0000000000000006
[   24.921803] RDX: 0000000000000007 RSI: 0000000000000096 RDI: ffff9bd9cfd16420
[   24.921804] RBP: ffff9bd9cfd03d28 R08: 0000000000000002 R09: 00000000000003a8
[   24.921805] R10: 0000002f485fca34 R11: 0000000000000000 R12: ffff9bd9bf043c1c
[   24.921806] R13: ffff9bd9c62fa3c0 R14: 0000000000000082 R15: 0000000000000000
[   24.921807] FS:  0000000000000000(0000) GS:ffff9bd9cfd00000(0000) knlGS:0000000000000000
[   24.921808] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   24.921808] CR2: 00007fb2648b0000 CR3: 0000000142c0a004 CR4: 00000000001606e0
[   24.921809] Call Trace:
[   24.921812]  <IRQ>
[   24.921819]  __usb_hcd_giveback_urb+0x8b/0x140
[   24.921821]  usb_hcd_giveback_urb+0xca/0xe0
[   24.921828]  xhci_giveback_urb_in_irq.isra.42+0x82/0xf0
[   24.921834]  handle_cmd_completion+0xe02/0x10d0
[   24.921837]  xhci_irq+0x274/0x4a0
[   24.921838]  xhci_msi_irq+0x11/0x20
[   24.921851]  __handle_irq_event_percpu+0x44/0x190
[   24.921856]  handle_irq_event_percpu+0x32/0x80
[   24.921861]  handle_irq_event+0x3b/0x5a
[   24.921867]  handle_edge_irq+0x80/0x190
[   24.921874]  handle_irq+0x20/0x30
[   24.921889]  do_IRQ+0x4e/0xe0
[   24.921891]  common_interrupt+0xf/0xf
[   24.921892]  </IRQ>
[   24.921900] RIP: 0010:usb_hcd_flush_endpoint+0x78/0x180
[   24.921354] usb 3-14: USB disconnect, device number 7

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>