ieee802154/atusb: Warn about outdated device firmware.

Together with mainlining the driver we released a first public binary version
of the device firmware. This is version 0.2. With this change we warn users
who run outdated versions of the firmware. While we have not seen problems
with it yet (thus no error, but a warning only) it would be better to run the
released and tested firmware. You can find released versions here:

http://downloads.qi-hardware.com/people/werner/wpan/web/



Signed-off-by: Stefan Schmidt <stefan@osg.samsung.com>
Acked-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>