iio: adc: ad7793: Add missing error code in ad7793_setup()

commit 4ed243b1 upstream.

Set error code while device ID query failed.

Fixes: 88bc3054

 ("IIO: ADC: New driver for AD7792/AD7793 3 Channel SPI ADC")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>