tumbler(4), snapper(4) and onyx(4) write the volume over I2C, and
iicbus_transfer() sleeps. This is why mixer_set() drops the mixer lock
around MIXER_SET() for non-MPSAFE drivers, relying on Giant to keep them
serialized.
Store the volume in the softc and let a task do the I2C write with no
lock held, so that the mixer method does not sleep at all. The lock
dropping for non-MPSAFE drivers will be removed in a follow-up patch.
Sponsored by: The FreeBSD Foundation
MFC after: 1 month