libata: remove superfluous NULL pointer checks

host->ports[] always contain pointers to valid port structures since
a "dummy port" structure is used in case if there is no physical port.

This patch takes care of two entries from Dan's list:

drivers/ata/sata_sil.c +535 sil_interrupt(13) warning: variable derefenced before check 'ap'
drivers/ata/sata_mv.c +2517 mv_unexpected_intr(6) warning: variable derefenced before check 'ap'

and of another needless NULL pointer check in pata_octeon_cf.c.

Reported-by: Dan Carpenter <error27@gmail.com>
Cc: corbet@lwn.net
Cc: eteo@redhat.com
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
diff --git a/drivers/ata/sata_mv.c b/drivers/ata/sata_mv.c
index 23714ae..c19417e 100644
--- a/drivers/ata/sata_mv.c
+++ b/drivers/ata/sata_mv.c
@@ -2514,7 +2514,7 @@
 	char *when = "idle";
 
 	ata_ehi_clear_desc(ehi);
-	if (!ap || (ap->flags & ATA_FLAG_DISABLED)) {
+	if (ap->flags & ATA_FLAG_DISABLED) {
 		when = "disabled";
 	} else if (edma_was_enabled) {
 		when = "EDMA enabled";