b43: update some transmit header constants

Some constants were missing and are added now.
The rate mode was renamed, at least the ucode in brcsmac uses 2 for HT
rates (iee80211n) and the more recent ucodes are using 3 for VHT
(ieee80211ac).

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>