[PATCH] ide: fix ide_toggle_bounce() to not try to bounce if we have an IOMMU

The following patch fixes a crash caused by attempting to bounce buffer
when an IDE CD-ROM is used on a machine with an IO-MMU. [At least, this
patch fixes things so I can use my IDE CD-ROM behind an ns87415 on a 
HP PA-RISC workstation.]

Signed-off-by: James Bottomley <jejb@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>