net: usbnet: make driver_info const

The driver_info field that is used for describing each of the usb-net
drivers using the usbnet.c core all declare their information as const
and the usbnet.c itself does not try and modify the struct.

It is therefore a good idea to make this const in the usbnet.c structure
in case anyone tries to modify it.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>