iio: ltc2983: fail probe if no channels are given

iio: ltc2983: fail probe if no channels are given

If there are no channels defined in the devicetree, there's no point in
probing the device. We were actually requesting a zero sized 'kmalloc'
array but since we were not touching the ZERO_SIZE_PTR afterwards,
nothing bad was actually happening. Hence this is not really a fix but
rather an improvement.

Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210825084149.11587-2-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>