ALSA: hda - Don't create unneeded digital input source for IDT 92HD71x

The current driver creates always the digital input source mixer
elements for IDT 92HD71x codecs no matter whether digital mics are
present.  This patch adds the proper check to avoid the creation of
these controls if unnecessary.

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