After commit b286f4e8 ("serial: core: Move tty and serdev to be
children of serial core port device") x86_instantiate_serdev() no longer
works due to the serdev-controller-device moving in the device hierarchy
from (e.g.) /sys/devices/pci0000:00/8086228A:00/serial0 to
/sys/devices/pci0000:00/8086228A:00/8086228A:00:0/8086228A:00:0.0/serial0

Use the new get_serdev_controller() helper function to fix this.

Fixes: b286f4e8 ("serial: core: Move tty and serdev to be children of serial core port device")
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20240216201721.239791-4-hdegoede@redhat.com