scsi: make ppa depend on !HIGHMEM

scsi: make ppa depend on !HIGHMEM

This is one of the last drivers depending on the block layer bounce
buffering code.  Restrict it to run on non-highmem configs so that the
bounce buffering code can be removed.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20250505081138.3435992-4-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>