ASoC: rsnd: adg: supply __printf(x, y) formatting for dbg_msg()

Fixes the following W=1 kernel build warning(s):

sound/soc/sh/rcar/adg.c: In function 'dbg_msg':
sound/soc/sh/rcar/adg.c:594:2: warning: function 'dbg_msg' might \
 be a candidate for 'gnu_printf' format attribute\
 [-Wsuggest-attribute=format]

Fixes: 1f9c82b5

 ("ASoC: rsnd: add debugfs support")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87tumhi21r.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>