ALSA: ice1724 - Check for ac97 to avoid kernel oops

Cards with identical PCI ids but no AC97 config in EEPROM do not have
the ac97 field initialized. We must check for this case to avoid kernel oops.

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