iio: adc: rcar-gyroadc: Remove devm_iio_device_alloc() error printing

devm_iio_device_alloc() can only fail due to a memory or IDA allocation
failure.  Hence there is no need to print a message, as the memory
allocation or IIO core code already takes care of that.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>