iio: adc: ad7949: enable use with non 14/16-bit controllers

iio: adc: ad7949: enable use with non 14/16-bit controllers

This driver supports devices with 14-bit and 16-bit sample sizes.
This implies different SPI transfer lengths which are not always handled
properly by some SPI controllers.

To work around this limitation, define a big endian buffer used to split
the buffer into two 8-bit messages in the event that the controller
doesn't support 14-bit or 16-bit transfers.
A separate buffer is introduced here to avoid performing operations on
types of different endianness.

Since all transfers use the same bits_per_word value, move that logic to
the probe function, and let transfers default to the value defined in
the struct spi_device.

Signed-off-by: Liam Beguin <lvb@xiphos.com>
Link: https://lore.kernel.org/r/20210815213309.2847711-3-liambeguin@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>