ath9k: backport dynack improvements

ath9k: backport dynack improvements

Close cooperation with Lorenzo Bianconi resulted
in these patches which fix all remaining seen issues
when using dynack.

Fix link losses when:
- Late Ack's are not seen or not present
- switching from too low static coverage class to dynack on a live link

These are fixed by setting the Ack Timeout/Slottime to
the max possible value for the currently used channel width when
a new station has been discovered.

When traffic flows, dynack is able to adjust to optimal values
within a few packets received (typically < 1 second)

These changes have been thoroughly tested on ~60 offshore devices
all interconnected using mesh over IBSS and dynack enabled on all.

Distances between devices varied from <100m up to ~35km

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
(cherry picked from commit f6e8ba0238fe349b7529357793e2fb18635819ed)