There are currently no provisions in place to ensure that the scanning
task has been stopped when the interface is stopped or removed.

This can result in a WARNING at net/wireless/core.c:643 and other badness
when you remove the module while a scan is happening.

Terminate the scanning task during interface stop.
Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>