ASoC: wm8998: Correct handling of input muxes

Currently, wm8998 has two input mux controls on IN1 and attempts to
switch these together when the A position is configured to be in digital
mode. This is because the digital mode requires pins from both the L and
R channels. However, this doesn't work as intended because whilst the
registers on the chip are changed the corresponding DAPM
representation is only updated for the mux actually being changed by the
user. The DAPM graph being out of sync with the hardware can cause some
odd issues with incorrect things being powered etc.

To avoid this issue and simplify the code somewhat, simply let the user
set the muxes as they desire. If they set an invalid configuration they
might not get audio from the DMIC but most of the chip requires you to
set a valid audio route to get audio.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>