alsa-mixer: Improve workaround for Valgrind's false warnings

alsa-mixer: Improve workaround for Valgrind's false warnings

Valgrind is not correctly handling ALSA TLV syscalls, which leads
to false warnings, looking like this:
 "Conditional jump or move depends on uninitialised value(s)"

Unfortunately, alsa-lib itself also uses these values which valgrind
falsely believe are uninitialized, so not all warnings are removed,
but this is what we can do from PA until the valgrind bug is fixed.

Signed-off-by: David Henningsson <david.henningsson@canonical.com>