iio: adc: rockchip_saradc: add NULL check on of_match_device() return value

iio: adc: rockchip_saradc: add NULL check on of_match_device() return value

Check return value from call to of_match_device()
in order to prevent a NULL pointer dereference.

In case of NULL print error message and return -ENODEV

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>