tty: serial: exar: constify pci_device_id.

tty: serial: exar: 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
   4030	   1280	      0	   5310	   14be	tty/serial/8250/8250_exar.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   4958	    352	      0	   5310	   14be	tty/serial/8250/8250_exar.o

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