auxdisplay: constify charlcd_ops.

charlcd_ops are not supposed to change at runtime. All functions
working with charlcd_ops provided by <misc/charlcd.h> work with
const charlcd_ops. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
  12750	    560	    362	  13672	   3568	drivers/auxdisplay/panel.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  12942	    368	    362	  13672	   3568	drivers/auxdisplay/panel.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>