net: ec_bhf: 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
   5113	    384	      0	   5497	   1579	drivers/net/ethernet/ec_bhf.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   5177	    320	      0	   5497	   1579	drivers/net/ethernet/ec_bhf.o
Signed-off-by: NArvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/ec_bhf.c

@@ -73,7 +73,7 @@
 
 #define ETHERCAT_MASTER_ID	0x14
 
-static struct pci_device_id ids[] = {
+static const struct pci_device_id ids[] = {
 	{ PCI_DEVICE(0x15ec, 0x5000), },
 	{ 0, }
 };