[ALSA] emu10k1 - Fix memory corruption

The number of mixer elements for SPDIF control don't match with the
actual array size (3).  This may result in a memory corruption that
overwrites the i2c_capture_source field (ALSA bug#3095).

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>