_id bits
14 and 15 set and other bits appearing random. Setting duration_id at driver to
zero results zd1211 outputting sane duration_id. This means that firmware is
setting correct values itself and expects duration_id to be zero in first
place.

Looking at vendor driver shows that only PSPoll frames have duration_id set by
driver, for other frames duration_id left zero.

Original bug-report and attached patch at:
  http://sourceforge.net/mailarchive/message.php?msg_id=28759111

Reported-by: Tomas Vanek <Tomas.Vanek@fbl.cz>
[modified original patch from bug-report, added check for pspoll frame]
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
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