iio: adc: checking for NULL instead of IS_ERR() in probe

mcb_request_mem() returns an ERR_PTR(), it doesn't return NULL.

Fixes: 74aeac4d ('iio: adc: Add MEN 16z188 ADC driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Stable@vger.kernel.org