staging: iio: ad5933: fix type mismatch regression

Commit 4c3577db ("Staging: iio: impedance-analyzer: Fix sparse
warning") fixed a compiler warning, but introduced a bug that resulted
in one of the two 16 bit IIO channels always being zero (when both are
enabled).

This is because int is 32 bits wide on most architectures and in the
case of a little-endian machine the two most significant bytes would
occupy the buffer for the second channel as 'val' is being passed as a
void pointer to 'iio_push_to_buffers()'.

Fix by defining 'val' as u16. Tested working on ARM64.

Fixes: 4c3577db

 ("Staging: iio: impedance-analyzer: Fix sparse warning")
Signed-off-by: David Schiller <david.schiller@jku.at>
Link: https://lore.kernel.org/r/20240122134916.2137957-1-david.schiller@jku.at
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>