backports: do not use DMA SG on old kernel versions

the usb net driver now uses the dma sg api which is not available on
old kernels, this patch removed the usage of the dma sg api for old
kernel versions.

This backports the following upstream commit:
commit 638c5115a794981441246fa8fa5d95c1875af5ba
Author: Ming Lei <ming.lei@canonical.com>
Date:   Thu Aug 8 21:48:24 2013 +0800

    USBNET: support DMA SG