iwlagn: set the sequence control from the transport layer

Since all the queue logic has been moved to the transport layer,
the sequence number is set in the transport layer.
While doing that I forgot that the mac header is copied to the
TB of the TX cmd in the upper layer before the call to the transport
layer. So basically we used the sequence number from mac80211...
This was fine for the first assocation but after the second, mac80211
resets its counters while we don't hence a shift that led to terrible
impact on performance.

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>