source/sink: Fix wrong calculation of thread_info.current_hw_volume
In pa_{source,sink}_new() and pa_{source,sink}_put() the current hardware
volume was miscalculated:
hw volume (dB) = real volume (dB) + soft volume (dB)
was used instead of
hw volume (dB) = real volume (dB) - soft volume (dB)
This lead to a crash in pa_alsa_path_set_volume() if high volumes were
set and the port was changed.
This patch fixes the calculation. Thanks to Tanu for pointing out
the correct solution.
link: https://bugs.freedesktop.org/show_bug.cgi?id=65520