iio: adc: stm32-dfsdm: fix st,adc-alt-channel property handling

iio: adc: stm32-dfsdm: fix st,adc-alt-channel property handling

Initially st,adc-alt-channel property was defined as an enum in the DFSDM
binding. The DFSDM binding has been changed to use the new IIO backend
framework, along with the adoption of IIO generic channels.
In this new binding st,adc-alt-channel is defined as a boolean property,
but it is still handled has an enum in DFSDM driver.
Fix st,adc-alt-channel property handling in DFSDM driver.

Fixes: 3208fa0cd919 ("iio: adc: stm32-dfsdm: adopt generic channels bindings")
Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>