usbnet: fix leak of transfer buffer of dev->interrupt

The transfer buffer of dev->interrupt is allocated in .probe path,
but not freed in .disconnet path, so mark the interrupt URB as
URB_FREE_BUFFER to free the buffer when the URB is destroyed.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Acked-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>