[net80211] add VHT ioctl parameters and driver capabilities

• Add the VHT capability element to the driver capabilities so ifconfig can see if VHT is available
• Add ioctl plumbing for enabling/disabling VHT and each of the VHT widths.

Note: this DOES change the ABI (the driver caps ioctl struct size, sigh)
so this will require a recompile of at least ifconfig.