cfg80211_ch_switch_notify uses ASSERT_WDEV_LOCK to assert that
net_device->ieee80211_ptr->mtx (which is the same as priv->wdev.mtx)
is held during the function's execution.
mwifiex_dfs_chan_sw_work_queue is one of its callers, which does not
hold that lock, therefore violating the assertion.
Add a lock around the call.

Disclaimer:
I am currently working on a static analyser to detect missing locks.
This was a reported case. I manually verified the report by looking
at the code, so that I do not send wrong information or patches.
After concluding that this seems to be a true positive, I created
this patch.
However, as I do not in fact have this particular hardware,
I was unable to test it.
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Niels Dossche <dossche.niels@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220321225515.32113-1-dossche.niels@gmail.com