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 <[e-mail]>
Reviewed-by: Geert Uytterhoeven <[e-mail]>
Signed-off-by: Andy Shevchenko <[e-mail]>

drivers/auxdisplay/img-ascii-lcd.c

···
 #endif
 }
 
-static struct img_ascii_lcd_config boston_config = {
+static const struct img_ascii_lcd_config boston_config = {
 	.num_chars = 8,
 	.ops = {
 		.update = boston_update,
···
 		pr_err_ratelimited("Failed to update LCD display: %d\n", err);
 }
 
-static struct img_ascii_lcd_config malta_config = {
+static const struct img_ascii_lcd_config malta_config = {
 	.num_chars = 8,
 	.external_regmap = true,
 	.ops = {
···
 		pr_err_ratelimited("Failed to update LCD display: %d\n", err);
 }
 
-static struct img_ascii_lcd_config sead3_config = {
+static const struct img_ascii_lcd_config sead3_config = {
 	.num_chars = 16,
 	.external_regmap = true,
 	.ops = {