serial: core: clear freed pointers on uart_register_driver() failure

In the Linux kernel, the following vulnerability has been resolved: serial: core: clear freed pointers on uart_register_driver() failure uart_register_driver() leaves drv->state pointing to freed memory when tty_alloc_driver() fails. If tty_register_driver() fails, drv->tty_driver also retains a pointer after its reference is dropped. Drivers that use drv->state as an "already registered" flag can then skip registration on the next probe and pass the freed state to uart_add_one_port(). This issue was found with failslab on QEMU's raspi1ap board by failing registration and binding the PL011 port again. Clear both pointers on their failure paths, as uart_unregister_driver() already does.