wifi: mac80211_hwsim: avoid mac80211 warning on bad rate

If the tool on the other side (e.g. wmediumd) gets confused
about the rate, we hit a warning in mac80211. Silence that
by effectively duplicating the check here and dropping the
frame silently (in mac80211 it's dropped with the warning).

Reported-by: Sönke Huster <shuster@seemoo.tu-darmstadt.de>
Tested-by: Sönke Huster <shuster@seemoo.tu-darmstadt.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>