mac80211: fix parsing of 40Mhz in injected radiotap header

The MCS bandwidth part of the radiotap header is 2 bits wide. The full 2
bit have to compared against IEEE80211_RADIOTAP_MCS_BW_40 and not only if
the first bit is set. Otherwise IEEE80211_RADIOTAP_MCS_BW_40 can be
confused with IEEE80211_RADIOTAP_MCS_BW_20U.

Fixes: dfdfc2be ("mac80211: Parse legacy and HT rate in injected frames")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>