iio: adc: adc128s052: Rename channel structs

iio: adc: adc128s052: Rename channel structs

The adc128s052 can be used to drive a few other ADCs but the TI's ADCs
it was originally intended for. The TI's IC variants model numbers don't
(trivially) explain the channel configuration (for a reader working with
other than TI's ICs).

Rename the channel configuration structures to explicitly explain they
are used for simple ADCs, having 2, 4 or 8 channels.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/e737f2b416e25d8e4e734e2765b0e21a3f0ae0bb.1755504346.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>