Fix wrong selection of internal clock when mclk is defined.

Resolve a logical inversion introduced in c9ec2cb3.

Fixes: c9ec2cb3 ("iio: adc: ad7192: use devm_clk_get_optional() for mclk")
Signed-off-by: Fabrizio Lamarque <fl.scratchpad@gmail.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Cc: <Stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20230530075311.400686-3-fl.scratchpad@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>