USB-Audio: Add second S/PDIF device on Phiree U2

USB-Audio: Add second S/PDIF device on Phiree U2

Phiree U2 has an unusual configuration. It only has S/PDIF output, but
there are still two devices presented:
- device 0: PCM audio, subject to volume control
- device 1: non-PCM data (passthrough), not subject to volume control

It looks like the AES bits are set according to the selected device,
since outputting PCM data via device 1 will not work (silence).

Currently only the device 0 is shown via the "iec958" alias, and the
second device is not accessible via hinted aliases.

Simply provide access to both of these devices via the "iec958" alias.

Reported-by: touc @ XBMC forum
Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: Takashi Iwai <tiwai@suse.de>