ipw2200: expire and use oldest BSS on adhoc create

If there are no networks on the free list, expire the oldest one when
creating a new adhoc network.  Because ipw2200 and the ieee80211 stack
don't actually cull old networks and place them back on the free list
unless they are needed for new probe responses, over time the free list
would become empty and creating an adhoc network would fail due to the !
list_empty(...) check.

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