iommu/dma: Fix NEED_SG_DMA_LENGTH dependency

IOMMU_DMA does indeed depend on scatterlists having a DMA length, but
the NEED_SG_DMA_LENGTH symbol should be selected, not depended upon.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>