source/sink: Fix wrong calculation of thread_info.current_hw_volume

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