wifi: wfx: scan_lock is global to the device

Currently, one scan_lock is associated to each vif. However, concurrent
scan on vifs is explicitly prohibited by the device. Currently,
scan_lock is associated with a vif but it is always locked with
conf_mutex (there is a case where conf_mutex is not associated to
scan_lock but scan_lock is tested on all interfaces). So concurrent scan
on vifs cannot happen.

So, this patch relocate scan_lock to the device and simplify the code.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231004172843.195332-7-jerome.pouiller@silabs.com