CVE-2023-54220 serial: 8250: Fix oops for port->pm on uart_change_pm()

In the Linux kernel, the following vulnerability has been resolved: serial: 8250: Fix oops for port-pm on uartchangepm Unloading a hardware specific 8250 driver can produce error "Unable to handle kernel paging request at virtual address" about ten seconds after unloading the driver. This happens...