iio: adc: ad4000: Avoid potential double data word read

Currently, SPI-Engine offload module always sends 32-bit data elements to
DMA engine. Appropriately, when set for SPI offloading, the IIO driver uses
32 storagebits for IIO ADC channel buffer elements. However, setting SPI
transfer length according to storagebits (32-bits in case of offload) can
lead to unnecessarily long transfers for ADCs that are 16-bit or less
precision. Adjust AD4000 single-shot read to run transfers of 2 bytes when
that is enough to get all ADC data bits.

Fixes: 59b51edf ("iio: adc: ad4000: Add support for SPI offload")
Suggested-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/8f765cfd6e93fad4e755dd95d709b7bea2a388e2.1744718916.git.marcelo.schmitt@analog.com


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