iwlagn: upper layer uses slabs to allocate tx cmds

In a near future, the upper layer won't be aware of the tx queues.
This allows to remove one place where the upper layer needed to
provide the tx queue index to the transport layer.
This also saves around 1.5MB.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>