iio: imu: adis16400: use DEFINE_DEBUGFS_ATTRIBUTE instead of DEFINE_SIMPLE_ATTRIBUTE

debugfs_create_file_unsafe does not protect the fops handed to it
against file removal. DEFINE_DEBUGFS_ATTRIBUTE makes the fops aware of
the file lifetime and thus protects it against removal.

Signed-off-by: Rohit Sarkar <rohitsarkar5398@gmail.com>
Tested-by Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>