iio: adc: ad7192: Simplify using devm_clk_get_optional_enabled()

If st->mclk is not NULL, then st->clock_sel is either AD7192_CLK_EXT_MCLK2
or AD7192_CLK_EXT_MCLK1_2.

So devm_clk_get_optional_enabled() can be used instead of hand writing it.
This saves some line of code.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/7dbe973905f1fdae5d2f5ae5a3b01dd1d6a9925b.1686774340.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>