libertas: harden-up exit paths

These simple sanity check avoids extra complexity in error paths when
moving to asynchronous firmware loading (which means the device may fail to
init some time after its creation).

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