[PATCH] ipw2200: enable wireless extension passive scan

This patch enables the ipw2200 driver to support passive scanning as
offered by the wireless extensions. For this, I enhanced the ipw_wx_set_scan
function in such a way that it differentiates between a passive and an
active scan request. Additionally, I added a new function called
ipw_request_passive_scan that is similiar to the ipw_request_scan
function to perform passive scans. Last but not least, I added a field
(in fact it is a work_struct struct) called request_passive_scan to
the ipw_priv struct.

Signed-off-by: Thomas King <king@informatik.uni-mannheim.de>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>