rt2x00: Implement HT protection for rt2800

Update the HT operation mode when mac80211 sends it to us and set
the different HT protection modes and rates accordingly. For now
only use CTS-to-self with OFDM 24M or CCK 11M when protection is
required.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>