wlcore: Load the NVS file asynchronously

The NVS file is loaded by the device's probe callback with the help of
request_firmware(). Since request_firmware() relies on udevd, the
modules cannot be loaded before hotplug events are handled.

Fix this by loading the NVS file asynchronously and continue
initialization only after the firmware request is over.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>