ath10k: use workqueue to set WEP TX key

Recent WMI/HTC changes made it possible for WMI
commands to sleep (if there's not enough HTC TX
credits to submit a command). TX path is in an
atomic context so calling WMI commands in it is
wrong.

This simply moves WEP key index update to a worker
and fixes the 'scheduling while atomic' bug.

This still leaves multiple WEP key handling laggy,
i.e. some frames may be TXed with an old/different
key (although recipient should still be able to RX
them).

kvalo: changed the title

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