serial: core: fix NULL pointer dereference in serial_core_unregister_port()

In the Linux kernel, the following vulnerability has been resolved:

serial: core: fix NULL pointer dereference in serial_core_unregister_port()

port->port_dev is NULL when no port device is installed: it is cleared
on teardown, and never set if registration failed before
serial_core_port_device_add(). serial_core_unregister_port() passes it
straight to serial_core_get_ctrl_dev(), which dereferences it:

KASAN: null-ptr-deref in range [0x0000000000000040-0x0000000000000047]
RIP: serial_core_unregister_port
Call Trace:
serial8250_unregister_port
serial8250_remove
unbind_store

Return early when there is no port device, and read port->port_dev
under port_mutex.

Also clear port->port_dev on the serial_core_register_port() error
path, where the port device has already been removed.