spa: alsa: Read ctl events instead of doing a global diff

This does a couple of things: first, we implement revents demangling, which seems to be required (although hw: devices work fine without it).

The second is to actually read the ctl events so we can tell when elements we care about have changed, instead of reading everything and trying to do a diff.

The latter is also required from a correctness perspective, as otherwise the ctl might keep triggering wakeups while the fd is ready to be read.