nl80211: sanitize array index in parse_txq_params

Wireless drivers rely on parse_txq_params to validate that
txq_params->ac is less than NL80211_NUM_ACS by the time the low-level
driver's ->conf_tx() handler is called. Use a new helper, 'array_idx',
to sanitize txq_params->ac with respect to speculation. I.e. ensure that
any speculation into ->conf_tx() handlers is done with a value of
txq_params->ac that is within the bounds of [0, NL80211_NUM_ACS).

Reported-by: Christian Lamparter <chunkeey@gmail.com>
Reported-by: Elena Reshetova <elena.reshetova@intel.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: linux-wireless@vger.kernel.org
Signed-off-by: Dan Williams <dan.j.williams@intel.com>