orinoco: abort scan on interface down

This fixes the problem causing the following trace:

------------[ cut here ]------------
WARNING: at linux-2.6.34/net/wireless/core.c:633 wdev_cleanup_work+0xb7/0xe0 [cfg80211]()
Hardware name: Latitude C840
Pid: 707, comm: cfg80211 Not tainted 2.6.34.7-0.5-desktop #1
Call Trace:
 [<c02065c3>] try_stack_unwind+0x173/0x190
 [<c02051cf>] dump_trace+0x3f/0xe0
 [<c020662b>] show_trace_log_lvl+0x4b/0x60
 [<c0206658>] show_trace+0x18/0x20
 [<c064e0b3>] dump_stack+0x6d/0x72
 [<c02443ae>] warn_slowpath_common+0x6e/0xb0
 [<c0244403>] warn_slowpath_null+0x13/0x20
 [<e2db5497>] wdev_cleanup_work+0xb7/0xe0 [cfg80211]
 [<c025cfa9>] run_workqueue+0x79/0x170
 [<c025d123>] worker_thread+0x83/0xe0
 [<c025fef4>] kthread+0x74/0x80
 [<c0203826>] kernel_thread_helper+0x6/0x10
---[ end trace 3f0348b3b0c6f4ff ]---

Reported by: Giacomo Comes <comes@naic.edu>
Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>