If the driver reports the rx timestamp at PLCP start, mac80211 can
only handle legacy encoding, but the code checks that the encoding
is not legacy. Fix this.

Fixes: da6a4352 ("mac80211: separate encoding/bandwidth from flags")
Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>