ASoC: renesas: fsi: Constify struct fsi_stream_handler

ASoC: renesas: fsi: Constify struct fsi_stream_handler

'struct fsi_stream_handler' is not modified in this driver.

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

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  51837	  12312	     64	  64213	   fad5	sound/soc/renesas/fsi.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  52125	  12024	     64	  64213	   fad5	sound/soc/renesas/fsi.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://patch.msgid.link/88ca34df9006b74a7596b91714e700bcff666c4b.1761507792.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Mark Brown <broonie@kernel.org>