ALSA: hda - Add expose_enum_ctl flag to snd_hda_add_vmaster_hook()

Since it's not always safe to assume that the vmaster hook is purely the
mute-LED control, add the flag indicating whether to expose the mute-LED
enum control or not.  Currently, conexant codec sets this off for non-HP
laptops where EAPD may be used really as EAPD.

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