rt2x00: don't use TXDONE_FALLBACK as success indicator

TXDONE_FALLBACK doesn't express if the frame was sent successful or not. It
only tells us that the hw used fallback rates for retries. Hence, don't use
TXDONE_FALLBACK as success indicator.

Before this patch we reported success to the rate control algorithm which
was wrong in a number of cases and might have lead to improper tx rate
selections.

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