ath9k: fix tx queue index confusion in debugfs code

Various places printing tx queue information used various different ways to
get a tx queue index for printing statistics. Most of these ways were wrong.

ATH_TXQ_AC_* cannot be used as an index for sc->tx.txq, because it is only
used internally for queue assignment.

One place used WME_AC_* as a queue index for sc->debug.stats.txstats, however
this array uses the ath9k_hw queue number as well.

Fix all of this by always using the ath9k_hw queue number as an index, and
always looking it up by going through sc->tx.txq_map.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>