Staging: iio/dac/ad5360.c: signedness bug in ad5360_read_raw()

ad5360_get_channel_vref() returns an int and scale_uv should be the
same.  Making it unsigned here breaks the error handling.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>