This patch addresses a long standing issue of the driver with the
mac80211 .flush() callback. Since implementing the .flush() callback
a number of issues have been fixed, but a WARN_ON_ONCE() was still
triggered because the timeout on the flush could still occur.

This patch changes the awkward design using msleep() into one using
a waitqueue. The waiting flush() context will kick the transmit dma
when it is idle and the timeout used waiting for the event is set
to 500 ms. Worst case there can be 64 frames outstanding for transmit
in the driver. At a rate of 1Mbps that would take 1.5 seconds assuming
MTU is 1500 bytes and ignoring retries. The WARN_ON_ONCE() is also
removed as this was put in to indicate the flush timeout as a reason
for the driver to stall. That was not happening since fixing endless
AMPDU retries with following upstream commit:

commit 85091fc0a75653e239dc8379658515e577544927
Author: Arend van Spriel <arend@broadcom.com>
Date:   Thu Feb 23 18:38:22 2012 +0100

    brcm80211: smac: fix endless retry of A-MPDU transmissions

bugzilla: 42840 <https://bugzilla.kernel.org/show_bug.cgi?id=42840>
bugzilla@redhat: <https://bugzilla.redhat.com/show_bug.cgi?id=799168>
bugzilla@redhat: <https://bugzilla.redhat.com/show_bug.cgi?id=787649>

Cc: Jonathan Nieder <jrnieder@gmail.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Camaleón <noelamac@gmail.com>
Cc: Milan Bouchet-Valat <nalimilan@club-internet.fr>
Cc: Seth Forshee <seth.forshee@canonical.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>