iio: inv_mpu6050: Check WHO_AM_I register on probe

This can be used to distinguish mpu6500. This is a warning rather than
an error because the differences are mostly irrelevant and it's nice to
avoid breaking users with slightly incorrect ACPI/DT.

Signed-off-by: Crestez Dan Leonard <leonard.crestez@intel.com>
Acked-by: Ge Gao <ggao@invensense.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>