This patch makes enum from the defines previously dwelled inside
ieee80211_tx_control for better readability.
The patch also addes HT flags, for 802.11n drivers:
- IEEE80211_TXCTL_OFDM_HT: request low-level driver to use HT OFDM rates
- IEEE80211_TXCTL_GREEN_FIELD: use green field protection
- IEEE80211_TXCTL_DUP_DATA: duplicate data on both 20 Mhz channels
- IEEE80211_TXCTL_40_MHZ_WIDTH: send this frame in 40Mhz width
- IEEE80211_TXCTL_SHORT_GI: send this frame with short guard interval

Tx command can be a combination of any of these flags, along with
bitrate represented by ieee80211_rate. this will allow legacy drivers to
switch easily to any 11n rate representation.

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
CC: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>