iwlwifi: arrange max number of Tx queues

This patch increases the max possible number of Tx queues, but leaves
current used number of queues as HW dependent

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>