iwlagn: set tx_fifo for ampdu in transport layer

the mapping tx_queue -> fifo is really transport related. The upper
layer should be involved in such things.

Note that upon agg_disable, the queue is always mapped to fifo 0, but
this doesn't matter since when the queue will be setup again for a
new BA session, it will be configured to the good fifo anyway.

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>