iio: adc: stm32-dfsdm: handle label as an optional property

The label property is defined as optional in the DFSDM binding.
Parse the label property only when it is defined in the device tree.

Fixes: 3208fa0c ("iio: adc: stm32-dfsdm: adopt generic channels bindings")
Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Link: https://patch.msgid.link/20241114102459.2497178-1-olivier.moysan@foss.st.com


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