alsa-mixer: Add "Headphone Mic" support for 3-pin ASUS netbooks
Some ASUS netbooks, such as the 1015 CX, have only one 3.5 mm jack,
but it can be used either as a headphone or as a mic (but not both
simultaneously).
This patch adds support for the "Headphone Mic" path that is used
on these devices, so that we can use the jack as an external mic, and
doing so without muting the speaker.
BugLink: https://bugs.launchpad.net/bugs/1018262
Signed-off-by: David Henningsson <david.henningsson@canonical.com>