swiotlb: clean up swiotlb_tbl_unmap_single

Remove a layer of pointless indentation, replace a hard to follow
ternary expression with a plain if/else.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jianxiong Gao <jxgao@google.com>
Tested-by: Jianxiong Gao <jxgao@google.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>