wcn36xx: pass correct BSS index when deleting BSS keys

The firmware message to delete BSS keys expects a BSS index to be passed.
This field is currently hard-coded to 0. Fix this by passing in the index
we received from the firmware when the BSS was configured.

The encryption type in that message also needs to be set to what was used
when the key was set, so the assignment of vif_priv->encrypt_type is now
done after the firmware command was sent. This reportedly fixes the
following error in AP mode:

  wcn36xx: ERROR hal_remove_bsskey response failed err=6

Also, AFAIU, when a BSS is deleted, the firmware apparently drops all the
keys associated with it. Trying to remove the key explicitly afterwards
will hence lead to the following message:

  wcn36xx: ERROR hal_remove_bsskey response failed err=16

This is now suppressed with an extra check for the BSS index validity.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>