alsa-mixer: When figuring out the max_dB of a path, use only channels that are used...

alsa-mixer: When figuring out the max_dB of a path, use only channels that are used by the path elements.

Without this, p->max_dB could never be less than 0 dB, because the loop at the
end of pa_alsa_path_probe() would reset p->max_dB to 0 as soon as the loop
encountered a channel that wasn't touched by any element.

There was a similar issue for p->min_dB too (it could never be more than 0 dB),
which is also fixed by this patch.