iio: proximity: isl29501: fix buffered read on big-endian systems

Fix passing a u32 value as a u16 buffer scan item. This works on little-
endian systems, but not on big-endian systems.

A new local variable is introduced for getting the register value and
the array is changed to a struct to make the data layout more explicit
rather than just changing the type and having to recalculate the proper
length needed for the timestamp.

Fixes: 1c287992 ("iio: light: isl29501: Add support for the ISL29501 ToF sensor.")
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250722-iio-use-more-iio_declare_buffer_with_ts-7-v2-1-d3ebeb001ed3@baylibre.com


Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>