alsa-ucm-conf: fix the rk817 playback mixer element

[pkilar]

Summary

• What is the goal of this PR? Make the volume control actually work on RK3326 handhelds using the rk817 codec.

The rk817-sound UCM profile sets:

PlaybackMixerElem "Master Playback Volume"

PlaybackMixerElem takes the simple mixer element name — Master — not the raw kcontrol name. Everywhere else in alsa-ucm-conf it is spelled that way (Headphone, Speaker, HP); rk817-sound is the only file using the long form.

So the element is never found, pipewire gets no hardware volume control, and its sink volume controls nothing audible — while common/050-audio deliberately pins ALSA Master to 100%. The symptom is a device that comes up at full volume after a cold boot while the stored setting reads whatever the user had chosen, and where changing the volume moves the number without moving the sound.

Testing

• How was this tested? Built and run on a Gusgu H7 (RK3326, rk817 codec).
• Test results: With the fix the pipewire sink drives the hardware on the dB curve — sink 0.30 → Master 67%, 0.20 → 56%. Before the fix Master never moved from 100% regardless of the sink value. Confirmed audible.

Additional Context

• This is a two-line change to an existing patch directory (patches/RK3326/), so it only affects RK3326.
• I deliberately left CaptureMasterElem "Master Capture Volume" alone. It looks like exactly the same error, but I have no way to test capture on this device and did not want to change what I could not verify — worth a look from someone who can.
• I only have one rk817 device. If the volume behaviour above is familiar on other RK3326 handhelds, this is likely the cause there too; a second data point would be welcome.

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AI Usage

Did you use AI tools to help write this code? YES — Claude Code was used for the investigation and to draft the patch and this description. The root cause was confirmed by reading the alsa-ucm-conf tree for how PlaybackMixerElem is spelled elsewhere, and the fix was verified against amixer readings on hardware.