iio: adc: ad7173: simplify clock enable/disable

iio: adc: ad7173: simplify clock enable/disable

Use devm_clk_get_enabled() instead of devm_clk_get(),
clk_prepare_enable(), devm_add_action_or_reset() to simplify the
code as it effectively does the same thing.

We can also drop ext_clk from struct ad7173_state since it is not used
anywhere else.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250620-iio-adc-ad7173-simplify-clock-enable-disable-v1-1-8bc693b190ec@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>