auxdisplay: img-ascii-lcd: Constify struct img_ascii_lcd_config

'struct img_ascii_lcd_config' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   6110	    728	      0	   6838	   1ab6	drivers/auxdisplay/img-ascii-lcd.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   6198	    632	      0	   6830	   1aae	drivers/auxdisplay/img-ascii-lcd.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>