wlcore: cancel channel switch work on interface removal

Otherwise, if the work is pending, we might get
a bad dereference after the interface is removed.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>