ipw2200: queue direct scans

When another scan is in progress, a direct scan gets dropped on the
floor.  However, that direct scan is usually the scan that's really
needed by userspace, and gets stomped on by all the broadcast scans the
ipw2200 driver issues internally.  Make sure the direct scan happens
eventually, and as a bonus ensure that the passive scan worker is
cleaned up when appropriate.

The change of request_passive_scan form a struct work to struct
delayed_work is only to make the set_wx_scan() code a bit simpler, it's
still only used with a delay of 0 to match previous behavior.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>