ASoC: blackfin: Fix machine driver Kconfig dependencies

Since the machine driver selects the CODEC driver we need to make sure that the
machine driver is only selectable if the CODEC driver can be build. This avoids
build errors under some configurations (which typically only result from
randconfig).

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