ALSA: hda - Fix jack-detection control of VT1708

VT1708 has no support for unsolicited events per jack-plug, the driver
implements the workq for polling the jack-detection.  The mixer element
"Jack Detect" was supposed to control this behavior on/off, but this
doesn't work properly as is now.  The workq is always started and the
HP automute is always enabled.

This patch fixes the jack-detect control behavior by triggering / stopping
the work appropriately at the state change.  Also the work checks the
internal state to continue scheduling or not.

Cc: <stable@kernel.org> [v3.1]
Signed-off-by: Takashi Iwai <tiwai@suse.de>