staging: iio: adis16060_core: Fix error handling

We need to check adis16060_spi_read return code to avoid feeding user
space with bogus data.
While, at it introduce out_unlock label in order to simplify locking
on error path.

Signed-off-by: Cristina Moraru <cristina.moraru09@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>