ASoC: audio-iio-aux: Use flex array to simplify code

"io-channel-names" is expected to have few values, so there is no real
point to allocate audio_iio_aux_chan structure with a dedicate memory
allocation.

Using a flexible array for struct audio_iio_aux->chans avoids the
overhead of an additional, managed, memory allocation.

This also saves an indirection when the array is accessed.

Finally, __counted_by() can be used for run-time bounds checking if
configured and supported by the compiler.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/1c0090aaf49504eaeaff5e7dd119fd37173290b5.1695540940.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Mark Brown <broonie@kernel.org>