ASoC: rt1318: Constify struct reg_sequence

'struct reg_sequence' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

While at it, remove rt1318_INIT_REG_LEN which is ununsed.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  22062	   4859	     32	  26953	   6949	sound/soc/codecs/rt1318.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  24742	   2171	     32	  26945	   6941	sound/soc/codecs/rt1318.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/96561dd2962d4312eb0e68ab850027f44350d070.1722952334.git.christophe.jaillet@wanadoo.frSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/codecs/rt1318.c

@@ -30,7 +30,7 @@
 
 #include "rt1318.h"
 
-static struct reg_sequence init_list[] = {
+static const struct reg_sequence init_list[] = {
 	{ 0x0000C000, 0x01},
 	{ 0x0000F20D, 0x00},
 	{ 0x0000F212, 0x3E},
@@ -254,7 +254,6 @@ static struct reg_sequence init_list[] = {
 	{ 0x0000C320, 0x20},
 	{ 0x0000C203, 0x9C},
 };
-#define rt1318_INIT_REG_LEN ARRAY_SIZE(init_list)
 
 static const struct reg_default rt1318_reg[] = {
 	{ 0xc000, 0x00 },