ALSA: HDA: Fix dock mic for Lenovo X220-tablet

Without the "thinkpad" quirk, the dock mic in
Lenovo X220 tablet edition won't work.

BugLink: http://bugs.launchpad.net/bugs/751033


Cc: stable@kernel.org
Tested-by: James Ferguson <james.ferguson@canonical.com>
Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>