iio: imu: adis16400: move adis16400.h into adis16400.c

Final part of the re-organization. Move the `adis16400.h` into
`adis16400.c` since it no longer needs to be shared among other files.

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