monitor & softdsp: fix alsa property names and api string

The alsa.pcm devices currently use "device.api" = "alsa:pcm"
but then the node overrides this later to just "alsa", which
is confusing. For now, let's match "alsa:pcm" in the monitor,
even though this does not appear on the node later.

This commit fixes device preferences in wireplumber.conf,
which were broken because the alsa endpoint names were empty
of info about the alsa device