mwifiex: add support for Marvell USB8797 chipset

This patch supports Avastar 88W8797 chipset with USB interface.

The corresponding firmware image file is located at:
"mrvl/usb8797_uapsta.bin"

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Frank Huang <frankh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>