staging: wfx: fix case of lack of tx_retry_policies

staging: wfx: fix case of lack of tx_retry_policies

In some rare cases, driver may not have any available tx_retry_policies.
In this case, the driver asks to mac80211 to stop sending data. However,
it seems that a race is possible and a few frames can be sent to the
driver. In this case, driver can't wait for free tx_retry_policies since
wfx_tx() must be atomic. So, this patch fix this case by sending these
frames with the special policy number 15.

The firmware normally use policy 15 to send internal frames (PS-poll,
beacons, etc...). So, it is not a so bad fallback.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191217161318.31402-3-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>