mwifiex: Change default firmware for PCIe8997 chipset

PCIe-USB8997 variant is being used in the product. Let's change default
firmware from PCIe-UART to PCIe-USB. So by default PCIe-USB firmware would
be downloaded if version register doesn't give any information.

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