ASoC: Properly handle spitz MIC GPIO

This patch firstly restructurizes the code a bit by getting rid of continuous
checking for machine type in spitz_mic_bias().

Then the patch properly requests the MIC GPIO in the spitz_init() and frees it
in spitz_exit().

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>