Pausing queue without checking threshold is racy with txdone path.
Moreover we do not need pause queue on any error, but only if queue
is full - in case when we send RTS frame ( other cases of almost full
queue are already handled in rt2x00queue_write_tx_frame() ).

Patch fixes of theoretically possible problem of pausing empty
queue.
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Tested-by: Enrico Mioso <mrkiko.rs@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>