PCI/PTM: Save PTM state in the saved capability store

PTM Control 寄存器改存偏移索引 store，pci_ptm_init() 调用 pci_reserve_ptm_state() 预留并报告失败；原先忽略 pci_add_ext_cap_save_buffer() 返回值，分配失败会静默丢失 PTM 状态。