ASoC: atmel-pdmic: mark symbols static where possible

We get 1 warning about global functions without a declaration
in the ASoC atmel-pdmic driver when building with W=1:
sound/soc/atmel/atmel-pdmic.c:286:5: warning: no previous prototype for 'pdmic_get_mic_volsw' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is declared
and don't need a declaration, but can be made static.
so this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>