iio: proximity: sx9500: use stack allocated struct for scan data

iio: proximity: sx9500: use stack allocated struct for scan data

Use a stack-allocated struct in sx9500_trigger_handler() to hold the
IIO buffer scan data. Since the scan buffer isn't used outside of this
function, it doesn't need to be in struct sx9500_data.

By always allocating enough space for the maximum number of channels,
we can avoid having to reallocate the buffer each time buffered reads
are enabled.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250722-iio-use-more-iio_declare_buffer_with_ts-4-v2-1-9e566f3a4c6a@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>