swiotlb-xen: replace dma_length with sg_dma_len() macro

swiotlb-xen has an implicit dependency on CONFIG_NEED_SG_DMA_LENGTH.
Remove it by replacing dma_length with sg_dma_len.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>