iio: magnetometer: support for lsm303dlh

The LSM303DLH accelerometer/magnetometer has a different
device identification method than using register 0x0f, instead
three registers contain a magic value. We rely on WhoAmI
to be zero for this variant.

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