[SCSI] Small cleanups for scsi_host.h

Small cleanups in scsi_host.h. Few #defines make me wonder if their
description is still up to date..?

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>