net: cadence: macb: constify pci_device_id.

pci_device_id are not supposed to change at runtime. All functions
working with pci_device_id provided by <linux/pci.h> work with
const pci_device_id. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
    791	    336	      0	   1127	    467	net/ethernet/cadence/macb_pci.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
    855	    272	      0	   1127	    467	net/ethernet/cadence/macb_pci.o
Signed-off-by: NArvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/cadence/macb_pci.c

@@ -128,7 +128,7 @@ static void macb_remove(struct pci_dev *pdev)
 	clk_unregister(plat_data->hclk);
 }
 
-static struct pci_device_id dev_id_table[] = {
+static const struct pci_device_id dev_id_table[] = {
 	{ PCI_DEVICE(CDNS_VENDOR_ID, CDNS_DEVICE_ID), },
 	{ 0, }
 };