When the hp mic pin has no VREF bits, the driver forgot to set PIN_IN
bit.  Spotted during debugging old MacBook Airs.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=65681
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>