mwifiex: Fix firmware filename for sd8977 chipset

Firmware for sd8977 chipset is distributed by Marvell package and also as
part of the linux-firmware repository in filename sdsd8977_combo_v2.bin.

This patch fixes mwifiex driver to load correct firmware file for sd8977.

Fixes: 1a0f5478 ("mwifiex: add support for sd8977 chipset")
Signed-off-by: Pali Rohár <pali@kernel.org>
Acked-by: Ganapathi Bhat <ganapathi.bhat@nxp.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>