SPARC64: fix iommu sg chaining

Commit 2c941a20 looks incomplete. The
helper functions like prepare_sg() need to support sg chaining too.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>