ALSA: hda - Fix input source elements of secondary ADCs on Realtek

Since alc_auto_create_input_ctls() doesn't set the elements for the
secondary ADCs, "Input Source" elemtns for these also get empty, resulting
in buggy outputs of alsactl like:
	control.14 {
		comment.access 'read write'
		comment.type ENUMERATED
		comment.count 1
		iface MIXER
		name 'Input Source'
		index 1
		value 0
	}

This patch fixes alc_mux_enum_*() (and others) to fall back to the
first entry if the secondary input mux is empty.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Cc: <stable@kernel.org>