mwifiex: Add USB8766 support

Adds new VID/PID for the Marvell 88W8766. The kernel currently only supports
the device as PCI, but it is also available using USB. An example of a device
delivered with the 88W8766 as USB is the Globalscale Mirabox.

TX buffer size is set to driver default (2K), as we do not know the firmware
requirement.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: John W. Linville <linville@tuxdriver.com>