mwifiex: PCIe8997 chip specific handling

The patch corrects the revision id register and uses it along with
magic value and chip version registers to download appropriate firmware
image.

PCIe8997 Z chipset variant code has been removed, as it won't be used in
production.

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