linux-firmware: add firmware for atusb driver

The atusb kernel driver is sitting in the bluetooth-next tree waiting for the
merge window to open. It's scheduled for 4.2.

Flashing goes into permanent storage and the GPLv2+ firmware allows for further
explorations. See ChangeLog for more details and links.

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: Kyle McMartin <kyle@kernel.org>