rt2x00: Improve cooperation between rt2800pci and minstrel

In order to lower the impact of probe rates don't send a frame as AMPDU
if the rate control algorithm sets IEEE80211_TX_CTL_RATE_CTRL_PROBE.
Otherwise a whole aggregate would be send with a probe rate which might
lead to numerous retries.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>