iio: adc: stm32: Fixing err code to not indicate success

[ Upstream commit 3735ca0b ]

This path would result in returning 0 / success on an error path.

Cc: Olivier Moysan <olivier.moysan@foss.st.com>
Fixes: 95bc8184 ("iio: adc: stm32-adc: add support of generic channels binding")
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Link: https://lore.kernel.org/r/20240330185305.1319844-4-jic23@kernel.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>