ASoC: adau1701: signedness bug in adau1701_write()

"ret" is supposed to be signed here.  The current code will only
return -EIO on error, instead of a more appropriate error code such
as -EAGAIN etc.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>