staging:iio:ad2s90: Make read_raw return spi_read's error code

Previously, when spi_read returned an error code inside ad2s90_read_raw,
the code was ignored and IIO_VAL_INT was returned. This patch makes the
function return the error code returned by spi_read when it fails.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>