'struct mtk_adsp_ipc_ops' is not modified in these drivers.

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

In order to do it, "struct mtk_adsp_ipc" also needs to be adjusted to this
new const qualifier.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  15533	   2383	      0	  17916	   45fc	sound/soc/sof/mediatek/mt8195/mt8195.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  15557	   2367	      0	  17924	   4604	sound/soc/sof/mediatek/mt8195/mt8195.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://msgid.link/r/a45d6b2b5ec040ea0fc78fca662c2dca3f13a49f.1718312321.git.christophe.jaillet@wanadoo.frSigned-off-by: Mark Brown <broonie@kernel.org>