tty: isicom: constify pci_device_id.

tty: isicom: 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
  12176	   1520	  25864	  39560	   9a88	drivers/tty/isicom.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  12528	   1168	  25864	  39560	   9a88	drivers/tty/isicom.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>