ALSA: hda - VIA: Fix VT1708 can't build up Headphone control issue

Since VT1708 didn't support the control of getting connection number,
building of headphone control will fail in via_hp_build() function.

Signed-off-by: Lydia Wang <lydiawang@viatech.com.cn>
Cc: <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>