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>

drivers/tty/serial/8250/8250_exar.c

@@ -601,7 +601,7 @@ static const struct exar8250_board pbn_exar_XR17V8358 = {
 		(kernel_ulong_t)&bd			\
 	}
 
-static struct pci_device_id exar_pci_tbl[] = {
+static const struct pci_device_id exar_pci_tbl[] = {
 	CONNECT_DEVICE(XR17C152, UART_2_232, pbn_connect),
 	CONNECT_DEVICE(XR17C154, UART_4_232, pbn_connect),
 	CONNECT_DEVICE(XR17C158, UART_8_232, pbn_connect),