ALSA: ASoC: Make WM8510 microphone input a DAPM mixer

The WM8510 microphone input PGA was represented as a DAPM PGA but in
DAPM terms the functionality is that of a mixer since it takes three
switchable inputs and produces one output. Representing it as an input
was causing its controls to be misinterpreted as gain controls and
would cause some required DAPM updates to be missed.

Reported-by: Jukka Hynninen <ext-jukka.hynninen@vaisala.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>