staging:iio:cdc:ad7150: Handle variation in chan_spec across device and irq present or not

The driver supports devices with different numbers of channels and
also can function without provision of an IRQ (with reduced features),
so this patch handles this cleanly by having multiple chan_spec
arrays and iio_info structures to pick between depending on what we
have.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210314181511.531414-7-jic23@kernel.org