iwlwifi: limit number of attempts for highest HT rate

When filling out its rate scale table, iwlwifi repeats the first HT rate
IWL_HT_NUMBER_TRY times. The hardware scheduler will stop using
aggregation for any frame that fails LINK_QUAL_AGG_DISABLE_START_DEF
times. Currently, both these constants equal 3.

If iwlwifi probes a faster rate than the link supports, all frames in a
(potentially tens of frames large) batch will fail IWL_HT_NUMBER_TRY
times. Because this happens to be as large as
LINK_QUAL_AGG_DISABLE_START_DEF, all frames will then be sent
individually. This leads to a short, but performance-degrading window
where the legacy stop-and-wait MAC takes over.

Bounding the initial rate by (LINK_QUAL_AGG_DISABLE_START_DEF-1)
attempts makes the third try use a lower rate and hence more be likely
to succeed. This somewhat mitigates the above described behavior.

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>

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