iwlwifi: mvm: update the firmware API in TX

The firmware team is now re-using a bit that hasn't been
used for a few generations. Re-use for TX_ON_AIR drop.
This bit will be set by the firmware to indicate that
a frame in an A-MPDU was dropped but not because of the
already mapped reasons.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>