alsa-mixer: Disable headphones when playing to speakers
Assume that the headphone port volume is lower than the speaker volume.
When plugging in headphones, if the path is active, while the jack is
being inserted and before it is actually detected as being plugged in,
it will still receive the signal being played (which is at a higher
volume than it will be when plugged in completely). The volume
difference manifests as a volume spike when the headphones are plugged
in, before the final volume is set.
This patch is required to prevent such a volume spike when plugging in
headphones. The problem is not fixed completely, but the spike is
shortened. To be fixed completely, we need to apply the port volume
before unmuting the new path.