This is a workaround for VHT-enabled STAs which break the spec
and have the VHT-MCS Rx map filled in with value 3 for all eight
spacial streams, an example is AR9462 in AP mode.

As per spec, in section 22.1.1 Introduction to the VHT PHY
A VHT STA shall support at least single spactial stream VHT-MCSs
0 to 7 (transmit and receive) in all supported channel widths.

Some devices in STA mode will get firmware assert when trying to
associate, examples are QCA9377 & QCA6174.

Packet example of broken VHT Cap IE of AR9462:

Tag: VHT Capabilities (IEEE Std 802.11ac/D3.1)
    Tag Number: VHT Capabilities (IEEE Std 802.11ac/D3.1) (191)
    Tag length: 12
    VHT Capabilities Info: 0x00000000
    VHT Supported MCS Set
        Rx MCS Map: 0xffff
            .... .... .... ..11 = Rx 1 SS: Not Supported (0x0003)
            .... .... .... 11.. = Rx 2 SS: Not Supported (0x0003)
            .... .... ..11 .... = Rx 3 SS: Not Supported (0x0003)
            .... .... 11.. .... = Rx 4 SS: Not Supported (0x0003)
            .... ..11 .... .... = Rx 5 SS: Not Supported (0x0003)
            .... 11.. .... .... = Rx 6 SS: Not Supported (0x0003)
            ..11 .... .... .... = Rx 7 SS: Not Supported (0x0003)
            11.. .... .... .... = Rx 8 SS: Not Supported (0x0003)
        ...0 0000 0000 0000 = Rx Highest Long GI Data Rate (in Mb/s, 0 = subfield not in use): 0x0000
        Tx MCS Map: 0xffff
        ...0 0000 0000 0000 = Tx Highest Long GI Data Rate  (in Mb/s, 0 = subfield not in use): 0x0000
Signed-off-by: Filip Matusiak <filip.matusiak@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>