ath10k: cleanup ath10k_bmi_fast_download()

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
ath10k: kill vdev_mtx and use conf_mutex

The vdev_mtx doesn't serve any purpose anymore.
The locking should be done with conf_mutex (it was
already, implicitly anyway).

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