watchdog: zx2967: constify zx2967_wdt_ops.

File size before:
   text	   data	    bss	    dec	    hex	filename
    988	    288	      0	   1276	    4fc	drivers/watchdog/zx2967_wdt.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   1084	    192	      0	   1276	    4fc	drivers/watchdog/zx2967_wdt.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>