bluetooth: btmrvl_sdio: look for sd8688 firmware in proper location

The firmware images are shared with libertas_sdio WiFi chip and used to be
in libertas/ subtree in linux-firmware. As btmrvl_sdio used to look into
the linux-firmware root, it ended up being unsuccessful. Since the
firmware files are not specific to the libertas hardware, they're being
moved into mrvl/ now.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>