iwlwifi: iwl_tx_queue_alloc : fix warning in printk formatting

This patch fix compilation warning in printk formatting
iwl_tx_queue_alloc function.
Cleanup the code a bit on the way.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>