net: usbnet: Remove unused driver_name variable

With GCC 6.3, we can get the following warning:

drivers/net/usb/usbnet.c:85:19: warning: 'driver_name' defined but not
used [-Wunused-const-variable=]
 static const char driver_name [] = "usbnet";
                   ^~~~~~~~~~~

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>