brcmsmac: fix tx queue flush infinite loop

This patch workaround live deadlock problem caused by infinite loop
in brcms_c_wait_for_tx_completion(). I do not consider the patch as
the proper fix, which should fix the real reason of tx queue flush
failure, but patch helps with system lockup.

Reference:
https://bugzilla.kernel.org/show_bug.cgi?id=42576



Reported-and-tested-by: Patrick <ragamuffin@datacomm.ch>
Cc: stable@vger.kernel.org # 3.2+
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>