[SPARC64]: Temporarily remove IOMMU merging code.

Changeset fde6a3c8

 ("iommu sg merging:
sparc64: make iommu respect the segment size limits") broke sparc64
because whilst it added the segment limiting code to the first pass of
SG mapping (in prepare_sg()) it did not add matching code to the
second pass handling (in fill_sg())

As a result the two passes disagree where the segment boundaries
should be, resulting in OOPSes, DMA corruption, and corrupted
superblocks.

Signed-off-by: David S. Miller <davem@davemloft.net>