iio: st_accel: support 8bit channel data

Some sensors like the LIS331DL only support 8bit data by a single
register per axis. These utilize the MSB byte. Make it possible
to register these apropriately.

A oneliner change is needed in the ST sensors core to handle 8bit
reads as this is the first supported 8bit sensor.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>