hostapd: Add vendor specific VHT extension for the 2.4 GHz band

hostapd: Add vendor specific VHT extension for the 2.4 GHz band

This allows vendor specific information element to be used to advertise
support for VHT on 2.4 GHz band. In practice, this is used to enable use
of 256 QAM rates (VHT-MCS 8 and 9) on 2.4 GHz band.

This functionality is disabled by default, but can be enabled with
vendor_vht=1 parameter in hostapd.conf if the driver advertises support
for VHT on either 2.4 or 5 GHz bands.

Signed-off-by: Yanbo Li <yanbol@qti.qualcomm.com>