ALSA: hda - Fix endless vmaster hook call in thinkpad_helper.c

The new vmaster hook, update_tpacpi_mute_led(), calls the original
vmaster hook, but I forgot to save the original hook function but keep
calling the updated one, which of course results in a stupid endless
loop.  Fixed now.

Signed-off-by: Takashi Iwai <tiwai@suse.de>