mac80211: drop frames for sta with no valid rate

When we're associated we should be able to send data to
target sta. If we cannot we may be trying to use the incorrect
band to talk to the sta. Lets catch any such cases, warn, and
drop the frames to not invalidate assumptions being made on
rate control algorithms when they have a valid sta to
communicate with. Any such cases should be handled and fixed.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>