ASoC: wm8904: implement input mode select as a mux

Make '* Capture Mode' a mux. This makes DAPM know that in single-ended
mode only inverting mux paths need to be enabled.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/125cd3c9f298da9b08a4d6002d4c00d70a898950.1566298834.git.mirq-linux@rere.qmqm.pl
Signed-off-by: Mark Brown <broonie@kernel.org>