iwlwifi: unite macros with same meaning

TFD_*_SLOTS and IWL_*_QUEUE_SIZE both define the TX queue
size (number of TFDs).
Get rid of TFD_*_SLOTS and use only IWL_*_QUEUE_SIZE.

Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>