cfg80211: add debugfs HT40 allow map

Here's a screenshot of what this looks like with ath9k:

mcgrof@pogo /debug/ieee80211/phy0 $ cat ht40allow_map
2412 HT40  +
2417 HT40  +
2422 HT40  +
2427 HT40  +
2432 HT40 -+
2437 HT40 -+
2442 HT40 -+
2447 HT40 -
2452 HT40 -
2457 HT40 -
2462 HT40 -
2467 Disabled
2472 Disabled
2484 Disabled
5180 HT40  +
5200 HT40 -+
5220 HT40 -+
5240 HT40 -+
5260 HT40 -+
5280 HT40 -+
5300 HT40 -+
5320 HT40 -
5500 HT40  +
5520 HT40 -+
5540 HT40 -+
5560 HT40 -+
5580 HT40 -+
5600 HT40 -+
5620 HT40 -+
5640 HT40 -+
5660 HT40 -+
5680 HT40 -+
5700 HT40 -
5745 HT40  +
5765 HT40 -+
5785 HT40 -+
5805 HT40 -+
5825 HT40 -

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>