libertas: add asynchronous firmware loading capability

As described at
http://article.gmane.org/gmane.linux.kernel.wireless.general/86084


libertas is taking a long time to load because it loads firmware
during module loading.

Add a new API for interface drivers to load their firmware
asynchronously. The same semantics of the firmware table are followed
like before.

Interface drivers will be converted in follow-up patches, then we can
remove the old, synchronous firmware loading function.

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