rt2x00: only set TXDONE_FALLBACK in rt2800pci if the frame was retried

TXDONE_FALLBACK expresses that fallback rates were used for retries. Hence,
it only makes sense to set the flag if retries > 0.

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>