btmrvl: Fix firmware filename for sd8997 chipset

Firmware for sd8997 chipset is distributed by Marvell package and also as
part of the linux-firmware repository in filename sdsd8997_combo_v4.bin.

This patch fixes mwifiex driver to load correct firmware file for sd8997.

Fixes: f0ef6748 ("Bluetooth: btmrvl: add sd8997 chipset support")
Signed-off-by: Pali Rohár <pali@kernel.org>
Acked-by: Ganapathi Bhat <ganapathi.bhat@nxp.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>