iio: adc: meson: move meson_sar_adc_set_chan7_mux routine upper

meson_sar_adc_set_chan7_mux is a basic func() for writing single register,
defined as static. Moved it up so it could be used in more places.

Signed-off-by: George Stark <GNStark@sberdevices.ru>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230714114010.293440-4-gnstark@sberdevices.ru
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>