ASoC: renesas: Fix non-static global variable

ASoC: renesas: Fix non-static global variable

When using global variables in a .c file only,it is necessary to add
the keyword "static", so here fix the warning.

sparse warnings: (new ones prefixed by >>)
>> sound/soc/renesas/dma-sh7760.c:62:3: sparse: sparse: symbol
'cam_pcm_data' was not declared. Should it be static?

Signed-off-by: songxiebing <songxiebing@kylinos.cn>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202412171210.7a4vH3Ew-lkp@intel.com/
Link: https://patch.msgid.link/20260325022804.253353-1-songxiebing@kylinos.cn
Signed-off-by: Mark Brown <broonie@kernel.org>