iio: adc: MCP3564: fix the static checker warning

iio: adc: MCP3564: fix the static checker warning

The patch 33ec3e5fc1ea: "iio: adc: adding support for MCP3564 ADC"
from Aug 29, 2023 (linux-next), leads to the following Smatch static
checker warning:

        drivers/iio/adc/mcp3564.c:1426 mcp3564_probe()
        warn: address of NULL pointer 'indio_dev'

drivers/iio/adc/mcp3564.c
    1421         struct iio_dev *indio_dev;
    1422         struct mcp3564_state *adc;
    1423
    1424         indio_dev = devm_iio_device_alloc(&spi->dev, sizeof(*adc));
    1425         if (!indio_dev) {
--> 1426                 dev_err_probe(&indio_dev->dev, PTR_ERR(indio_dev),
                                       ^^^^^^^^^^^^^^^

Fixes: 33ec3e5fc1ea (iio: adc: adding support for MCP3564 ADC)
Signed-off-by: Marius Cristea <marius.cristea@microchip.com>
Link: https://lore.kernel.org/r/20230918075633.1884-1-marius.cristea@microchip.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>