wl1251: Fix queue stopping/waking for TX path

This patch was adapted from 06f7bc7d
(from linus's linux-2.6 tree of kernel.org)

here's the original message:
    The queue stopping/waking functionality was broken in a way that could
    cause huge latencies in TX transfers and even cause the TX to stall in the
    right circumstances. Correct these problems.

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Acked-by: Kalle Valo <kvalo@adurom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>