iio: adis16480: Add debugfs_reg_access entry

The driver is defining debugfs entries by calling
`adis16480_debugfs_init()`. However, those entries are attached to the
iio_dev debugfs entry which won't exist if no debugfs_reg_access
callback is provided.

Fixes: 2f3abe6c ("iio:imu: Add support for the ADIS16480 and similar IMUs")
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>