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.fr


Signed-off-by: Mark Brown <broonie@kernel.org>