Fix CONFIG_AC97_BUS dependency

CONFIG_AC97_BUS is used from both sound and ucb1400 drivers.
The recent change in Kconfig introduced the exclusive dependency on
CONFIG_SOUND, and disabled the ucb1400 build without sound.
This patch makes CONFIG_AC97_BUS independent.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Randy Dunlap <randy.dunlap@oracle.com>