ASoC: codecs: wcd938x: fix mux error handling

ASoC: codecs: wcd938x: fix mux error handling

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.

Fixes: eec611d26f84 ("ASoC: codecs: wcd938x: add mux control support for hp audio mux")
Link: https://lore.kernel.org/lkml/Z-z_ZAyVBK5ui50k@hovoldconsulting.com/
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://patch.msgid.link/20250415074145.7202-2-johan+linaro@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>