ipw2x00: simplify scan_event handling

* Drop unnesssary delayd_work_pending() tests.

* Unify scan_event_{now|later} by using mod_delayed_work() w/ 0 delay
  for scan_event_now.

* Make ipw2200 scan_event handling match ipw2100 - use
  mod_delayed_work() w/ 0 delay for immediate scanning.

Only compile tested.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Stanislav Yakovlev <stas.yakovlev@gmail.com>
Cc: linux-wireless@vger.kernel.org