iwlagn: fix command queue timeout

If the command queue is constantly busy,
which can happen in P2P, the hangcheck
timer will frequently find a command in
it and will eventually reset the device
because nothing sets the timestamp for
this queue when commands are processed.

Fix this by setting the timestamp when
a command completes.

Cc: stable@kernel.org #2.6.39, #3.0.0 #3.1.0
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
SIgned-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>