iio: accel: kxsd9: Split out transport mechanism

Split off a transport mechanism struct that will deal with the SPI
traffic in preparation for adding I2C support.

Tested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>