iio: adc: adi-axi-adc: fix typo in doc-string

The channels are of type iio_chan_spec, not axi_adc_chan_spec. They were in
some earlier version, but forgot to rename in the doc-string.

Fixes: ef040706 ("iio: adc: adi-axi-adc: add support for AXI ADC IP core")
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210219090134.48057-1-alexandru.ardelean@analog.com


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