iio: imx7d_adc: Remove unneeded error message

In case of ioremap failure, the core code will take care of printing
the error message, so there is no need for having a local error
message in the driver.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>