iio: adc: ad4695: use u16 for buffer elements

Change the type of the buffer elements to u16 since we currently only
support 16-bit word size. The code was originally written to also allow
for 32-bit word size when oversampling is enabled, but so far,
oversampling is only implemented when using SPI offload and therefore
doesn't use this buffer.

AD4695_MAX_CHANNEL_SIZE macro is dropped since it no longer adds any
value.

AD4695_MAX_CHANNELS + 2 is changed to AD4695_MAX_CHANNELS + 1 because
previously we were overallocating. AD4695_MAX_CHANNELS is the number of
of voltage channels and + 1 is for the temperature channel.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Trevor Gamblin <tgamblin@baylibre.com>
Link: https://patch.msgid.link/20250428-iio-introduce-iio_declare_buffer_with_ts-v4-2-6f7f6126f1cb@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>