mac80211: Fix incorrect condition when checking rx timestamp

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>