iio: adc: ad7192: use devm_clk_get_optional() for mclk

iio: adc: ad7192: use devm_clk_get_optional() for mclk

The devm_clk_get_optional() helper returns NULL when devm_clk_get() returns
-ENOENT.
This makes things slightly cleaner. The added benefit is mostly cosmetic.

Also, a minor detail with this call, is that the reference for the parent
device is taken as `spi->dev` instead of `&st->sd.spi->dev` (which looks a
little quirky).

Signed-off-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210513120752.90074-10-aardelean@deviqon.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>