mac80211: parse the HT capabilities info through debugfs

When debugging you want to be lazy and not have to parse
bits yourself so let mac80211 debugfs do the parsing for you.

This is what I get against my WRT610N:

root@tux:~# cat /sys/kernel/debug/ieee80211/phy0/stations/00\:22\:6b\:aa\:bb\:01/ht_capa
ht supported
cap: 0x000e
	HT20/HT40
	SM Power Save disabled
	No RX STBC
	Max AMSDU length: 7935 bytes
	No DSSS/CCK HT40
ampdu factor/density: 2/6
MCS mask: ff ff 00 00 00 00 00 00 00 00
MCS rx highest: 0
MCS tx params: 0

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