ath9k: Clarify we only want 32-bit DMA

Use DMA_32BIT_MASK to clarify we only want 32-bit DMA
memory. What was there before is also 32-bit but this makes it
clearer
Signed-off-by: NLuis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath9k/main.c

@@ -1591,9 +1591,8 @@ static int ath_pci_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 	if (pci_enable_device(pdev))
 		return -EIO;
 
-	/* XXX 32-bit addressing only */
-	if (pci_set_dma_mask(pdev, 0xffffffff)) {
-		printk(KERN_ERR "ath_pci: 32-bit DMA not available\n");
+	if (pci_set_dma_mask(pdev, DMA_32BIT_MASK)) {
+		printk(KERN_ERR "ath9k: 32-bit DMA not available\n");
 		ret = -ENODEV;
 		goto bad;
 	}