ASoC: bfin-ac97: Fix prototype error following AC'97 refactoring

As part of the multiplatform refactoring for AC'97 the AC'97 bus ops were
staticised meaning that the prototype (which was never needed) conflicts
with the declaration causing build failures.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>