swiotlb: use unmap_single instead of swiotlb_unmap_single in swiotlb_free_coherent

We don't need any check in swiotlb_unmap_single here. unmap_single is
appropriate.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>