iwlwifi: mvm: correctly fill the ac array in the iwl_mac_ctx_cmd

The indexes into the ac array in the iwl_mac_ctx_cmd are from the iwl_ac
enum and not the txfs.  The current code therefore puts the edca params
in the wrong indexes of the array, causing wrong priority for
data-streams of different ACs.
Fix this.

Note that this bug only occurs in NICs that use the new tx api, since in
the old tx api the txf number is equal to the corresponding ac in the
iwl_ac enum.

Signed-off-by: Naftali Goldstein <naftali.goldstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>