iio: ad7949: fix incorrect SPI xfer len

This driver supports 14-bits and 16-bits devices. All of them have a 14-bit
configuration registers. All SPI trasfers, for reading AD conversion
results and for writing the configuration register, fit in two bytes.

The driver always uses 4-bytes xfers which seems at least pointless (maybe
even harmful). This patch trims the SPI xfer len and the buffer size to
two bytes.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>