ASoC: Use __printf markup to silence compiler

Silence warnings (triggered at W=1) by adding relevant __printf
attributes.

  sound/soc/soc-dapm.c:149:2: warning: function 'pop_dbg' might be a candidate for 'gnu_printf' format attribute [-Wsuggest-attribute=format]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Mark Brown <broonie@kernel.org>