mwifiex: firmware name correction for usb8997 chipset

Similar to pcie8997 chipset, first firmware submitted for usb8997
chipset will be usbusb8997_combo_v4.bin. This patch corrects the
name used in driver.

Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>