staging:iio:ad2s90: Make probe handle spi_setup failure

Previously, ad2s90_probe ignored the return code from spi_setup, not
handling its possible failure. This patch makes ad2s90_probe check if
the code is an error code and, if so, do the following:

- Call dev_err with an appropriate error message.
- Return the spi_setup's error code.

Note: The 'return ret' statement could be out of the 'if' block, but
this whole block will be moved up in the function in the patch:
'staging:iio:ad2s90: Move device registration to the end of probe'.
Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/staging/iio/resolver/ad2s90.c

@@ -86,7 +86,12 @@ static int ad2s90_probe(struct spi_device *spi)
 	/* need 600ns between CS and the first falling edge of SCLK */
 	spi->max_speed_hz = 830000;
 	spi->mode = SPI_MODE_3;
-	spi_setup(spi);
+	ret = spi_setup(spi);
+
+	if (ret < 0) {
+		dev_err(&spi->dev, "spi_setup failed!\n");
+		return ret;
+	}
 
 	return 0;
 }