[ALSA] emu10k1 - Fix handling of ac97_chip=2

EMU10K1/EMU10K2 driver
Fixed the handling of ac97_chip=2 capability type.
The error occurs in snd_ac97_mixer(), not in snd_ac97_bus().
Also, release the unnecessary ac97_bus object in the error path.

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