b43legacy: Work around mac80211 race condition

As shown in http://thread.gmane.org/gmane.linux.kernel.wireless.general/36497

,
mac80211 has a bug that allows a call to the TX routine after the queues have
been stopped. This situation will only occur under extreme stress. Although
b43legacy does not crash when this condition occurs, it does generate a WARN_ON
and also logs a queue overrun message. This patch recognizes b43legacy is not
at fault and logs a message only when the most verbose debugging mode is
enabled. In the unlikely event that the queue is not stopped when the DMA
queue becomes full, then a warning is issued.

This patch is based on the one used by b43.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>