source: Fix monitor source rate changing
When a sink changes its sample rate, also the monitor source rate
needs to be changed. In order to determine whether a source supports
rate changing, the code checks if the update_rate() callback is set,
but monitor sources don't have that callback set, so the old code
always failed to change the monitor source rate.
This patch fixes the monitor source rate changing by handling monitor
sources as a special case in pa_source_update_rate(): if the source is
a monitor source, then the update_rate() callback is not required.
BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=66424