iio: adc: mxs-lradc: Fix return value check in mxs_lradc_adc_probe()

In case of error, the function devm_ioremap() returns NULL pointer
not ERR_PTR(). The IS_ERR() test in the return value check should
be replaced with NULL test. Also add NULL test for iores.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>