ALSA: ice1724 - External clock item only for cards with SPDIF_IN

Append the external clock item to the clock list only if
the SPDIF_IN capability is defined in the SPDIF register.

Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>