ASoC: codecs: wcd938x: fix mux error handling

Merge series from Johan Hovold <johan+linaro@kernel.org>:

A recent change added support for looking up an optional mux control
before falling back to gpio control for us-euro plug selection.

The mux framework does however not yet support optional muxes and an
error message is now incorrectly logged on machines like the Lenovo
ThinkPad X13s which do not have one:

    wcd938x_codec audio-codec: /audio-codec: failed to get mux-control (0)

Suppress the bogus error and add the missing mux error handling by
making sure that the 'mux-controls' DT property is present before
looking up the mux control.

Included is also a related cleanup.