mwl8k: make initial firmware load asynchronous

Introduce a firmware loading state machine to manage the process
of loading firmware asynchronously and completing initialization
upon success.  The state machine attempts to load the preferred
firmware image.  If that fails, and if an alternative firmware
image is available, it will attempt to load that one.

Signed-off-by: Brian Cavagnolo <brian@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>