ALSA: hda - Make snd_hda_gen_spec_free() static

The last user of snd_hda_gen_spec_free() is patch_via.c, and we can
rewrite it safely with snd_hda_gen_free(), so that
snd_hda_gen_spec_free() can be a local function in hda_generic.c.

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