ALSA: HDA VIA: Fix crash on codecs without Headphone

Don't enumerate via_hp_mixer while hp_mux is null (headphone does not exist),
to fix the crash of via_independent_hp_info (via_hp_mixer's .info), which will
reference hp_mux.

Signed-off-by: Logan Li <LoganLi@viatech.com.cn>
Signed-off-by: Harald Welte <HaraldWelte@viatech.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>