ASoC: Fix typo in MPC5200 PSC AC97 driver Kconfig

ALSA SoC drivers should be specify SND_SOC_AC97_BUS instead, not AC97_BUS.
Without SND_SOC_AC97_BUS defined, an AC97 device will not get correctly
registered on the AC97 bus, which prevents thinks like the WM9712
touchscreen driver from getting probed.

Tested against 2.6.31-rc1.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Jon Smirl <jonsmirl@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>