alsa-mixer: Add support for the Center/LFE element

Merged Tanu Kaskinen requested to merge tanuk/pulseaudio:center-lfe into master

We already supported the CLFE element, which should be semantically equivalent, so I just copied all the CLFE element definitions.

The Center/LFE element is seen on Creative X-Fi with 20K1 chipset cards.

Fixes: #978 (closed)

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