iwlagn: reserve queue 10 for TX during scan dwell

New uCode images will use queue 10 for TX
during scan (for P2P offchannel operation
scan). We'll bump the API version of those,
but before we need to reserve queue 10 and
stop using it for aggregation.

To simplify the code, always reserve it,
we could continue using it on older uCode
images but that'd be rather complicated.
Also, we'll set it up to map to the right
FIFO as needed later, but as we don't use
the queue now that doesn't hurt.

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>