isdn: avm: c4: 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
  11803	    544	      1	  12348	   303c	isdn/hardware/avm/c4.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  11931	    416	      1	  12348	   303c	isdn/hardware/avm/c4.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>