iwlwifi: clean up queue/fifo handling

4965 hardware has 7 queues reserved and the
remaining ones used for aggregation, 5000
and higher need to have 10 reserved. This
is not very clear in the code right now,
unfortunately.

Introduce a new IWL_TX_FIFO_UNUSED constant
and make the queue/FIFO mapping arrays able
to hold that value, and change the setup
code to reserve all queues in the arrays
(the queue number is the index) and use the
new unused constant to not map those queues
to any FIFO.

Additionally, clear up the AC/queue mapping
code to be more understandable. The mapping
is the identity mapping right now, but with
the mapping function I think it's easier to
understand what happens there.

Finally, HCCA isn't implemented at all and
I think newer microcode removed it, so let's
remove all mention of it in the code, some
comments remain for 4965.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Shanyu Zhao <shanyu.zhao@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>