ata: fix sparse warning in libata.h

Avoids lots of these, also is more readable.
include/linux/libata.h:1210:13: warning: potentially expensive pointer subtraction

Change the subtraction to addition on the other side of the comparison.

Thanks to Christer Weinigel for the suggestion.
Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Acked-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

include/linux/libata.h

@@ -1197,7 +1197,7 @@ static inline struct ata_link *ata_port_next_link(struct ata_link *link)
 		return ap->pmp_link;
 	}
 
-	if (++link - ap->pmp_link < ap->nr_pmp_links)
+	if (++link < ap->nr_pmp_links + ap->pmp_link)
 		return link;
 	return NULL;
 }