iio: adc: Max9611: checking for ERR_PTR instead of NULL in probe

devm_iio_device_alloc() doesn't return ERR_PTRs, it returns NULLs.

Fixes: 69780a3b ("iio: adc: Add Maxim max9611 ADC driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>