iio: adc: ad7124: Remove input number limitation

The driver limits the user to use only 4/8 differential inputs, but this
device has the option to use pseudo-differential channels. This will
increase the number of channels to be equal with the number of inputs so 8
channels for ad7124-4 and 16 for ad7124-8.

This patch removes the check between channel nodes and num_inputs value.

Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>