mwifiex: Fix memory corruption in dump_station

The mwifiex_cfg80211_dump_station() uses static variable for iterating
over a linked list of all associated stations (when the driver is in UAP
role). This has a race condition if .dump_station is called in parallel
for multiple interfaces. This corruption can be triggered by registering
multiple SSIDs and calling, in parallel for multiple interfaces
    iw dev <iface> station dump

[16750.719775] Unable to handle kernel paging request at virtual address dead000000000110
...
[16750.899173] Call trace:
[16750.901696]  mwifiex_cfg80211_dump_station+0x94/0x100 [mwifiex]
[16750.907824]  nl80211_dump_station+0xbc/0x278 [cfg80211]
[16750.913160]  netlink_dump+0xe8/0x320
[16750.916827]  netlink_recvmsg+0x1b4/0x338
[16750.920861]  ____sys_recvmsg+0x7c/0x2b0
[16750.924801]  ___sys_recvmsg+0x70/0x98
[16750.928564]  __sys_recvmsg+0x58/0xa0
[16750.932238]  __arm64_sys_recvmsg+0x28/0x30
[16750.936453]  el0_svc_common.constprop.3+0x90/0x158
[16750.941378]  do_el0_svc+0x74/0x90
[16750.944784]  el0_sync_handler+0x12c/0x1a8
[16750.948903]  el0_sync+0x114/0x140
[16750.952312] Code: f9400003 f907f423 eb02007f 54fffd60 (b9401060)
[16750.958583] ---[ end trace c8ad181c2f4b8576 ]---

This patch drops the use of the static iterator, and instead every time
the function is called iterates to the idx-th position of the
linked-list.

It would be better to convert the code not to use linked list for
associated stations storage (since the chip has a limited number of
associated stations anyway - it could just be an array). Such a change
may be proposed in the future. In the meantime this patch can backported
into stable kernels in this simple form.

Fixes: 8baca1a3

 ("mwifiex: dump station support in uap mode")
Signed-off-by: Pali Rohár <pali@kernel.org>
Acked-by: Ganapathi Bhat <ganapathi.bhat@nxp.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200515075924.13841-1-pali@kernel.org