Support HT capability overrides

Support HT capability overrides

This allows HT capabilities overrides on kernels that
support these features.

MCS Rates can be disabled to force to slower speeds when using HT.
Rates cannot be forced higher.

HT can be disabled, forcing an 802.11a/b/g/n station to act like
an 802.11a/b/g station.

HT40 can be disabled.

MAX A-MSDU can be disabled.
A-MPDU Factor and A-MPDU Density can be modified.

Please note that these are suggestions to the kernel. Only mac80211
drivers will work at all. The A-MPDU Factor can only be decreased and
the A-MPDU Density can only be increased currently.

Signed-hostap: Ben Greear <greearb@candelatech.com>