iio: adc: ad7476: Drop convstart chan_spec

iio: adc: ad7476: Drop convstart chan_spec

The ad7476 driver defines separate chan_spec structures for operation
with and without convstart GPIO. At quick glance this may seem as if the
driver did provide more than 1 data-channel to users - one for the
regular data, other for the data obtained with the convstart GPIO.

The only difference between the 'convstart' and 'non convstart'
-channels is presence / absence of the BIT(IIO_CHAN_INFO_RAW) in
channel's flags.

We can drop the convstart channel spec, and related convstart macro, by
allocating a mutable per driver instance channel spec and adding the flag
in probe if needed. This will simplify the driver with the cost of added
memory consumption.

Assuming there aren't systems with very many ADCs and very few
resources, this tradeoff seems worth making.

Simplify the driver by dropping the 'convstart' channel spec and
allocating the channel spec for each driver instance.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/cd7c72e3ee00f279d3381873f54e0c5b75b5ad11.1754901948.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>