tty: serial: 8250: protect against NULL uart->port.dev in register

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

tty: serial: 8250: protect against NULL uart->port.dev in register

serial8250_register_8250_port() conditionally copies uart->port.dev
from up->port.dev only when up->port.dev is non-NULL:

if (up->port.dev) {
uart->port.dev = up->port.dev;
...
}

So if both the existing uart slot and up have a NULL ->dev,
uart->port.dev remains NULL. The very next ACPI companion check
then dereferences it unconditionally:

if (!has_acpi_companion(uart->port.dev)) {

has_acpi_companion() reads dev->fwnode without a NULL guard
(include/linux/acpi.h), so this NULL-derefs the kernel for the
remaining no-dev case rather than just skipping the
mctrl_gpio_init() initialisation as intended.

smatch flags the inconsistency:

drivers/tty/serial/8250/8250_core.c:767
serial8250_register_8250_port() error: 'uart->port.dev' could be
null (see line 719)

Guard the call with a NULL check so register continues to work
for callers that legitimately have no parent device (legacy
non-OF/non-ACPI registrations).

No functional change for callers that pass a non-NULL ->dev.