libertas: prioritize usb8388_olpc.bin firmware on OLPC machines

Normally, the v9 firmware will be loaded if it's available.  However, on
OLPC XO-1 machines, the olpc-specific firmware supports extra functionality.
This makes the libertas driver attempt to load the custom firmware first
if the machine is an OLPC machine; if that fails (or it's not an OLPC
machine), fall back to attempting to load the other firmwares.

usb8388_olpc.bin is currently found in the linux-firmware repository.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>