USBNET: support DMA SG

This patch introduces support of DMA SG if the USB host controller
which usbnet device is attached to is capable of building packet from
discontinuous buffers.

The patch supports passing the skb fragment buffers to usb stack directly
via urb->sg.

Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: Grant Grundler <grundler@google.com>
Cc: Freddy Xin <freddy@asix.com.tw>
Cc: Alan Stern <stern@rowland.harvard.edu>
Acked-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>