mac80211: fix legacy and invalid rx-rate report

This fixes obtaining the rate info via sta_set_sinfo
when the rx rate is invalid (for instance, on IBSS
interface that has received no frames from one of its
peers).

Also initialize rinfo->flags for legacy rates, to not
rely on the whole sinfo being initialized to zero.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>