rt2x00: rt2800usb: limit tx queues length

TX status fifo is limited to 16 elements. When we send more frames than
that, we can easily loose status, what is not good for rate scaling
algorithm.

On my testing the change does not degrade performance, actually make
is slightly better. Additionally with the patch I can see much less
various rt2x00 warnings in dmesg.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>