tty: mxser: 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
  20253	   1184	  19904	  41341	   a17d	drivers/tty/mxser.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  21117	    300	  19904	  41341	   a17d	drivers/tty/mxser.o

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