iwlwifi: memory allocation optimization

This patch optimizes memory allocation. The cmd member of
iwl_tx_queue was allocated previously as a continuous block
of memory. This patch allocates separate memory chunks for each command
and maps/unmaps these chunks in the run time.

Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>