net: lantiq_xrx200: add ingress SG DMA support

This patch adds support for scatter gather DMA. DMA in PMAC splits
the packet into several buffers when the MTU on the CPU port is
less than the MTU of the switch. The first buffer starts at an
offset of NET_IP_ALIGN. In subsequent buffers, dma ignores the
offset. Thanks to this patch, the user can still connect to the
device in such a situation. For normal configurations, the patch
has no effect on performance.

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>