swiotlb: use phys_to_dma and dma_to_phys

This converts swiotlb to use phys_to_dma and dma_to_phys instead of
swiotlb_phys_to_bus() and swiotlb_bus_to_phys().

swiotlb_phys_to_bus() and swiotlb_bus_to_phys() are not necessary so
this patch also removes them.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Becky Bruce <beckyb@kernel.crashing.org>