mt76: mt76u: rely only on data buffer for usb control messagges

mt76: mt76u: rely only on data buffer for usb control messagges

Starting from commit 'a6bfb6d13f33 ("mt76: usb: use max packet length
for m76u_copy")' reg_val does not share memory with usb data buffer.
On non-coherent devices this approach can corrupt data pointer since data
and reg_val share the same cache-line, resulting in the following crash:

[  371.544901] CPU 0 Unable to handle kernel paging request at virtual address 00000000, epc == 8042fbb0
[  371.558521] CPU: 0 PID: 11 Comm: kworker/u2:2 Not tainted 4.14.160 #0
[  371.565204] Workqueue: mt76u mt76u_deinit [mt76_usb]
[  371.570331] task: 83823ac0 task.stack: 8386c000
[  371.575004] $ 0   : 00000000 80590000 00000000 00000000
[  371.580407] $ 4   : 82edaad0 00000002 83823ac0 fffffff8
[  371.585810] $ 8   : fffffffd 0000fc00 8052da00 00000000
[  371.591212] $12   : 000b2285 ae53a1a9 00108845 89da44c4
[  371.596615] $16   : 82edaad0 82ed9d20 00001798 832edf00
[  371.602019] $20   : 00000000 8386dda8 80530000 fffffffe
[  371.607421] $24   : 8051d040 76274d1b
[  371.612824] $28   : 8386c000 8386dd88 82edaad4 830d4d50
[  371.618228] Hi    : 000000f7
[  371.621203] Lo    : 33333371
[  371.624196] epc   : 8042fbb0 __mutex_lock.isra.2+0x134/0x378
[  371.630043] ra    : 830d4d50 mt76u_deinit+0x418/0xa6c [mt76_usb]
[  371.636237] Status: 1000fc03KERNEL EXL IE
[  371.640557] Cause : 0080000c (ExcCode 03)
[  371.644696] BadVA : 00000000
[  371.647671] PrId  : 00019374 (MIPS 24Kc)
[  371.726123]  usbcore nls_base usb_common
[  371.730180] Process kworker/u2:2 (pid: 11, threadinfo=8386c000, task=83823ac0, tls=00000000)
[  371.738884] Stack : 833d009c 83210b4c 82ed8bc0 8386ddac 000001ff 8008ac50 8386ddac 83b74b00
[  371.747519]         82edaad4 00000000 83b74b48 83210c38 82edaad0 82ed9d20 00001798 832edf00
[  371.756157]         00000000 00000000 80530000 fffffffe 80530000 830d4d50 00000040 8389d850
[  371.764794]         8052d9d8 8389d850 8386de30 82ed9d20 8386de5f 831c27bc 833d48ec 8052d9d8
[  371.773431]         83823ac0 83823af0 82edab00 82ed9d20 8386de5f 831c5c30 00000000 8052d9a8
[  371.782069]         ...
[  371.784598] Call Trace:
[  371.787130] [<8042fbb0>] __mutex_lock.isra.2+0x134/0x378
[  371.792622] [<830d4d50>] mt76u_deinit+0x418/0xa6c [mt76_usb]
[  371.808546]
[  371.810920] ---[ end trace c62f0601f6730eb0 ]---
[  371.818101] Kernel panic - not syncing: Fatal exception
[  371.824420] Rebooting in 3 seconds..

Fix the issue relying only on data buffer to send/receive usb control messages

Fixes: a6bfb6d13f33 ("mt76: usb: use max packet length for m76u_copy")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>