iio: imu: adis16400: fix unprotected debugfs reads

In the Linux kernel, the following vulnerability has been resolved: iio: imu: adis16400: fix unprotected debugfs reads adis16400_serial_number_fops is open coded and never calls debugfs_file_get(), so debugfs_remove_recursive() does not wait for a read in progress and unbind frees the iio_dev underneath it. Commit ae1d37a9bb4b ("iio: imu: adis16400: use DEFINE_DEBUGFS_ATTRIBUTE instead of DEFINE_SIMPLE_ATTRIBUTE") converted product_id and flash_count to DEFINE_DEBUGFS_ATTRIBUTE, which protects itself, but moved all three files to debugfs_create_file_unsafe(). serial_number prints three registers as "%.4x-%.4x-%.4x" and DEFINE_DEBUGFS_ATTRIBUTE() formats a single u64, so it cannot be converted without changing what the file returns. adis16475 and adis16550 already use debugfs_create_file() for their open coded files. Use it here too.