libertas: fix changing interface type when interface is down

The recent changes to only power the device when the interface up
introduced a bug: changing interface type, legal when the interface
is down, performs device I/O.

Fix this functionality by validating and recording the interface
type when the change is requested, but only applying the change
if/when the interface is brought up.

Signed-off-by: Daniel Drake <dsd@laptop.org>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>