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_sil.c b/drivers/ata/sata_sil.c
index 030ec07..35bd5cc 100644
--- a/drivers/ata/sata_sil.c
+++ b/drivers/ata/sata_sil.c
@@ -532,7 +532,7 @@
 		struct ata_port *ap = host->ports[i];
 		u32 bmdma2 = readl(mmio_base + sil_port[ap->port_no].bmdma2);
 
-		if (unlikely(!ap || ap->flags & ATA_FLAG_DISABLED))
+		if (unlikely(ap->flags & ATA_FLAG_DISABLED))
 			continue;
 
 		/* turn off SATA_IRQ if not supported */