p54: fix crash during initialization

p54: fix crash during initialization

This patch fixes a crash that got introduced when the
mentioned patch replaced  the direct list_head access
with skb_peek_tail(). When the device is starting up,
there are  no entries in  the queue, so previously to
"Use skb_peek_tail() instead..." the target_skb would
end up as the  tail and head pointer which then could
be used by __skb_queue_after to fill the empty queue.

With skb_peek_tail() in its place will instead just
return NULL which then causes a crash in the
__skb_queue_after().

| BUG: unable to handle kernel NULL pointer dereference at 000000
| #PF error: [normal kernel read fault]
| PGD 0 P4D 0
| Oops: 0000 [#1] SMP PTI
| CPU: 0 PID: 12 Comm: kworker/0:1 Tainted: GO   5.1.0-rc7-wt+ #218
| Hardware name: MSI MS-7816/Z87-G43 (MS-7816), BIOS V1.11 05/09/2015
| Workqueue: events request_firmware_work_func
| RIP: 0010:p54_tx_pending+0x10f/0x1b0 [p54common]
| Code: 78 06 80 78 28 00 74 6d <48> 8b 07 49 89 7c 24 08 49 89 04 24 4
| RSP: 0018:ffffa81c81927d90 EFLAGS: 00010086
| RAX: ffff9bbaaf131048 RBX: 0000000000020670 RCX: 0000000000020264
| RDX: ffff9bbaa976d660 RSI: 0000000000000202 RDI: 0000000000000000
| RBP: ffff9bbaa976d620 R08: 00000000000006c0 R09: ffff9bbaa976d660
| R10: 0000000000000000 R11: ffffe8480dbc5900 R12: ffff9bbb45e87700
| R13: ffff9bbaa976d648 R14: ffff9bbaa976d674 R15: ffff9bbaaf131048
| FS:  0000000000000000(0000) GS:ffff9bbb5ec00000(0000) knlGS:00000
| CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
| CR2: 0000000000000000 CR3: 00000003695fc003 CR4: 00000000001606f0
| Call Trace:
|  p54_download_eeprom+0xbe/0x120 [p54common]
|  p54_read_eeprom+0x7f/0xc0 [p54common]
|  p54u_load_firmware_cb+0xe0/0x160 [p54usb]
|  request_firmware_work_func+0x42/0x80
|  process_one_work+0x1f5/0x3f0
|  worker_thread+0x28/0x3c0

Cc: stable@vger.kernel.org
Fixes: e3554197fc8f ("p54: Use skb_peek_tail() instead of direct head pointer accesses.")
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>