mac80211: Fix mac80211 station info rx bitrate for IBSS mode

Filter out incoming multicast packages before applying their bitrate
to the rx bitrate station info field to prevent them from setting the
rx bitrate to the basic multicast rate.

Signed-off-by: Henning Rogge <hrogge@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>