Allow modules to use it, fixing a build failure when the newly added
ADAU1977 driver is built as a module.
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NMark Brown <broonie@linaro.org>
Acked-by: NTakashi Iwai <tiwai@suse.de>