libertas sdio: fix suspend when interface is down

When the interface is down, the hardware is powered off.
However, the suspend handler currently tries to send host sleep commands
(when wakeup params are set) in this configuration, causing a system hang
when going into suspend (the commands will never complete).

Avoid this by detecting this situation and simply returning from
the suspend handler without doing anything special.

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