wcn36xx: Introduce mutual exclusion of fw configuration

As the association status changes the driver needs to configure the
hardware. This is done based on information in the "sta" acquired by
ieee80211_find_sta(), which requires the caller to ensure that the "sta"
is valid while its being used; generally by entering an rcu read
section.

But the operations acting on the "sta" has to communicate with the
firmware and may therefor sleep, resulting in the following report:

[   31.418190] BUG: sleeping function called from invalid context at
kernel/locking/mutex.c:238
[   31.425919] in_atomic(): 0, irqs_disabled(): 0, pid: 34, name:
kworker/u8:1
[   31.434609] CPU: 0 PID: 34 Comm: kworker/u8:1 Tainted: G        W
4.12.0-rc4-next-20170607+ #993
[   31.441002] Hardware name: Qualcomm Technologies, Inc. APQ 8016 SBC
(DT)
[   31.450380] Workqueue: phy0 ieee80211_iface_work
[   31.457226] Call trace:
[   31.461830] [<ffffff8008088c58>] dump_backtrace+0x0/0x260
[   31.464004] [<ffffff8008088f7c>] show_stack+0x14/0x20
[   31.469557] [<ffffff8008392e70>] dump_stack+0x98/0xb8
[   31.474592] [<ffffff80080e4330>] ___might_sleep+0xf0/0x118
[   31.479626] [<ffffff80080e43a8>] __might_sleep+0x50/0x88
[   31.485010] [<ffffff80088ff9a4>] mutex_lock+0x24/0x60
[   31.490479] [<ffffff8008595c38>] wcn36xx_smd_set_link_st+0x30/0x130
[   31.495428] [<ffffff8008591ed8>] wcn36xx_bss_info_changed+0x148/0x448
[   31.501504] [<ffffff80088ab3c4>]
ieee80211_bss_info_change_notify+0xbc/0x118
[   31.508102] [<ffffff80088f841c>] ieee80211_assoc_success+0x664/0x7f8
[   31.515220] [<ffffff80088e13d4>]
ieee80211_rx_mgmt_assoc_resp+0x144/0x2d8
[   31.521555] [<ffffff80088e1e20>]
ieee80211_sta_rx_queued_mgmt+0x190/0x698
[   31.528239] [<ffffff80088bc44c>] ieee80211_iface_work+0x234/0x368
[   31.535011] [<ffffff80080d81ac>] process_one_work+0x1cc/0x340
[   31.541086] [<ffffff80080d8368>] worker_thread+0x48/0x430
[   31.546814] [<ffffff80080de448>] kthread+0x108/0x138
[   31.552195] [<ffffff8008082ec0>] ret_from_fork+0x10/0x50

In order to ensure that the "sta" remains alive (and consistent) for the
duration of bss_info_changed() mutual exclusion has to be ensured with
sta_remove().

This is done by introducing a mutex to cover firmware configuration
changes, which is made to also ensure mutual exclusion between other
operations changing the state or configuration of the firmware. With
this we can drop the rcu read lock.

Cc: stable@vger.kernel.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>