Fix IDE CD-drive PIO mode.

Fix IDE CD-drive PIO mode.
CD drives in PIO mode don't work under Xen because of a change in Linux
between 2.6.12 and 2.6.16, as a result of the following thread:
http://lists.parisc-linux.org/pipermail/parisc-linux/2005-August/027197.html

The change breaks systems which have highmem and a swiotlb because the
ide-cd driver doesn't use the swiotlb, resulting in read/writes to/from
highmem pages in PIO mode not working any longer.  Xen kernels usually have
both highmem and a swiotlb.

Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>