iwlwifi: pcie: support short Tx queues for A000 device family

iwlwifi: pcie: support short Tx queues for A000 device family

This allows to modify TFD_TX_CMD_SLOTS to a power of 2
which is smaller than 256.
Note that we still need to set values to wrap at 256
into the scheduler's write pointer, but all the rest of
the code can use shorter transmit queues.

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