libertas: terminate scan when stopping interface

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>