ath10k: workaround fw beaconing bug

ath10k: workaround fw beaconing bug

Some firmware revisions don't wait for beacon tx
completion before sending another SWBA event. This
could lead to hardware using old (freed) beacon
data in some cases, e.g. tx credit starvation
combined with missed TBTT. This is very very rare.

On non-IOMMU-enabled hosts this could be a
possible security issue because hw could beacon
some random data on the air.  On IOMMU-enabled
hosts DMAR faults would occur in most cases and
target device would crash.

Since there are no beacon tx completions (implicit
nor explicit) propagated to host the only
workaround for this is to allocate a DMA-coherent
buffer for a lifetime of a vif and use it for all
beacon tx commands. Worst case for this approach
is some beacons may become corrupted, e.g. garbled
IEs or out-of-date TIM bitmap.

Keep the original beacon-related code as-is in
case future firmware revisions solve this problem
so that the old path can be easily re-enabled with
a fw_feature flag.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>