ASoC: use DEFINE_SHOW_ATTRIBUTE() to decrease code duplication

There is some duplicate code in soc-core.c, and the kernel provides
DEFINE_SHOW_ATTRIBUTE() helper macro to decrease it in seq_file.h.

Signed-off-by: Peng Donglin <dolinux.peng@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>