rt2x00: pause almost full queue early

Do not drop &queue->tx_lock and acquire it again to pause queue when
available entries are below the threshold.

Patch should mitigate problem of frequently printed errors:
"Error - Dropping frame due to full tx queue"

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Tested-by: Enrico <Mioso@gmail.com>
Tested-by: Enrico Mioso <mrkiko.rs@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>