iwlagn: move the disable agg logic to transport layer

Since all the check_empty logic is now in the transport layer,
the upper layer doesn't need to know anything about tx queues.
The disable aggregation flow was the last to know what a tx queue
is, so move it too.

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>