iw: add HT capability parsing for scanning

iw: add HT capability parsing for scanning

This adds HT capability IE parsing for scanning. You will now
be able to easily review what HT capabilities your AP supports
with a simple iw scan.

Sreenshot of a scan result of a 1-stream AP:

BSS 00:03:7f:12:34:56 (on wlan6)
        TSF: 1817911860 usec (0d, 00:30:17)
        freq: 2427
        beacon interval: 100
        capability: ESS ShortPreamble SpectrumMgmt ShortSlotTime (0x0521)
        signal: -60.00 dBm
        last seen: 11390 ms ago
        SSID: sucia-perra
        Supported rates: 1.0* 2.0* 5.5* 11.0* 6.0 9.0 12.0 18.0
        DS Parameter set: channel 4
        Power constraint: 0 dB
        ERP: <no flags>
        Extended supported rates: 24.0 36.0 48.0 54.0
        HT capabilities:
                Capabilities: 0x010c
                        HT20
                        SM Power Save disabled
                        RX STBC 1-stream
                        Max AMSDU length: 7935 bytes
                        No DSSS/CCK HT40
                Maximum RX AMPDU length 65535 bytes (exponent: 0x003)
                Minimum RX AMPDU time spacing: 1/2 usec (0x02)
                MCS set:  00 00 00 00 00 00 00 00 00 00 00 00 00 00 ff
                Supported RX MCS Indexes:
                        MCS Index 0
                        MCS Index 1
                        MCS Index 2
                        MCS Index 3
                        MCS Index 4
                        MCS Index 5
                        MCS Index 6
                        MCS Index 7
                No TX MCS set defined
        Extended capabilities: HT Information Exchange Supported
        WMM:    * Parameter version 1
                * u-APSD
                * BE: CW 15-1023, AIFSN 3
                * BK: CW 15-1023, AIFSN 7
                * VI: CW 7-15, AIFSN 2, TXOP 3008 usec
                * VO: acm CW 3-7, AIFSN 2, TXOP 1504 usec

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>