iio: st_sensors: make detection more helpful

The ST sensors are detected by reading a WhoAmI register and
matching the number found to a sensor name string. To make it
easier to figure out what happens when things go wrong, print
the WhoAmI value and the device name we're trying to match.

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