ALSA: hda - Fix silent headphone on Thinkpads with AD1984A codec

AD1984A codec has a couple of pins with EAPD controls, and the generic
codec driver tries to turn each of them on/off depending on the pin
active state.  However, Thinkpads seem to use EAPD of the speaker pin
as a master EAPD for controlling the mute of all outputs, including
the headphone.  This results in the dead headphone output via the
headphone plugging because it mutes the speaker and turns off EAPD.

The fix is to simply add spec->gen.keep_on_eapd flag.

[This is a regression fix on 3.12 where we moved the AD codec parser
 to the generic parser.  3.11 and earlier didn't show this problem
 because still static quirks have been used.]

Reported-and-tested-by: Vito Caputo <vcaputo@gnugeneration.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>