iwlagn: fix a race in the unmapping of the TFDs

While inspecting the code, I saw that iwl_tx_queue_unmap modifies
the read pointer of the Tx queue without taking any locks. This means
that it can race with the reclaim flow. This can possibly lead to
a DMA warning complaining that we unmap the same buffer twice.

This is more a W/A than a fix since it is really weird to take
sta_lock inside iwl_tx_queue_unmap, but it can help until we revamp
the locking model in the transport layer.

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>