ASoC: adau17x1: Unused exported functions changed to internal

adau17x1_setup_firmware and adau17x1_has_dsp is only used internally, so
making them static instead of exported.

Signed-off-by: Robert Rosengren <robertr@axis.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>