ARM: defconfig: fix CONFIG_SND_SOC_AC97_CODEC name

The CONFIG_SND_SOC_AC97 symbol was recently enabled but does not
actually exist. From the patch description, I assume that this
was meant to be CONFIG_SND_SOC_AC97_CODEC, which is used in
imx_v6_v7_defconfig.

Fixes: 2cc1cd26 ("ARM: configs: Enable ASoC AC'97 glue")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>