debugfs: remove return value of debugfs_create_x64()

No one checks the return value of debugfs_create_x64(), as it's not
needed, so make the return value void, so that no one tries to do so in
the future.

Link: https://lore.kernel.org/r/20191011132931.1186197-8-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>