[SCSI] ppa: fix for machines with highmem

ppa cannot handle highmem pages, and like imm, which already has
this patch, the device is slow, so performance is not a big issue,
so just force pages to be in low memory (hence mapped).

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>