rt2x00: Enable fallback rates for rt61pci and rt73usb

Explicitly enable the usage of fallback rates for
the transmission of frames with rt61pci and rt73usb hardware.

Note that for txdone reporting, only rt61pci is capable of
reporting the fallback rates, for USB it is not possible
to determine the number of retries. However the device will
use the fallback rates, so it might still help in the performance.

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