qla2xxx: Restrict max_lun to 16-bit for older HBAs

Older HBAs are only capable of supporting 16-bit LUNs,
so we need to make sure to adjust max_lun accordingly.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Acked-by: Chad Dupuis <chad.dupuis@qlogic.com>
Reviewed-by: Ewan Milne <emilne@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>