iwlagn: fix cmd queue unmap

When we stop the device while a command is in
flight that uses multiple TBs, we can leak the
DMA buffers for the second and higher TBs. Fix
this by using iwlagn_unmap_tfd() as we do when
we normally recover the entry.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>