iio: imu: adis: add support product ID check in adis_initial_startup

Each driver/chip that wants to validate it's product id, can now
specify a 'prod_id_reg' and an expected 'prod_id' value.
The 'prod_id' value is intentionally left 0 (uninitialized). There aren't
(yet) any product IDs with value 0; this enforces that both 'prod_id_reg'
and 'prod_id' are specified.

At the very least, this routine validates that the SPI connection to the
ADIS chip[s] works well.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>