fix(acpi_panasonic): acquire serial lock before calling hkey_sound_mute in shutdown

[guest-seuros]

Summary

acpi_panasonic_shutdown() calls hkey_sound_mute() without holding the ACPI serial lock.
Every other call site holds ACPI_SERIAL_BEGIN(panasonic) before calling it, as the locking model is caller-owned.

This seems to affect the whole Panasonic Toughtbook lineup btw.

Test Plan

Boot a Panasonic Notebook with acpi_panasonic loaded. Trigger a clean shutdown and reboot, verify no static audio burst through the speaker.

[ngie]

hkey_sound_mute calls ACPI_SERIAL_ASSERT(..), which I assume is where things panic. This is a pretty straightforward change.