alsa-mixer: refactor element_probe and fix >2 channel bug
By refactoring volume probing into its own function, we can reduce
indentation a lot. Also, if an error occurs during the volume probe,
that volume element is now always skipped (instead of taking down
the entire path with it).
Also, a bug for elements with more than two channels is fixed, as
previously, the volume parsing code was continuing, potentially
referencing somewhere outside the array (which has max two channels).
Signed-off-by: David Henningsson <david.henningsson@canonical.com>