blackfin: bf537: fix typo "CONFIG_SND_SOC_ADV80X_MODULE"

There's a (rather subtle) typo in "CONFIG_SND_SOC_ADV80X_MODULE". Fix it
once and for all by using IS_ENABLED(), which is designed to avoid
issues like this.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>