net: rmnet_usb: Index netdev name based on driver_info description

Normal RmNet functionality requires netdevice name to be
'rmnet_usb0'. While CV2X instance requires it to be 'rmnet_usb1'.
Currently, the driver assigns 'rmnet_usb0' name to CV2X function
if it is the only/first rmnet function being bound.
Make changes to add this indexing in the usbnet driver_info and
accordingly parse it to append at the end of 'rmnet_usb'.

Change-Id: I17fd390a22725017116e0eea5adf9c96506f30e6
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>