snd_maestro3: Do not hold sc_lock across the hwvol handlers

[christos]

Summary

mixer_hwvol_*() can end up taking sound(4)'s channel lock through
mixer_set_softpcmvol() or mixer_set_eq(). m3_pchan_trigger() takes
sc_lock with the channel lock already held, which can cause a LOR. Drop
sc_lock for the duration of the hwvol handlers, as snd_uaudio(4) does
for the same reason.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation