iwlwifi: pcie: dynamic Tx command queue size

Devices in the A000 family can use a different size for the command queue.
To allow this, make the command queue size configurable and set the size
for A000 devices to 32.

Signed-off-by: Shahar S Matityahu <shahar.s.matityahu@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>