efiserialio: use port settings (sio->Mode) for initial setup

Use serial port setup done by system firmware.
ARM64 Hyper-V does hung if we attempt to override the defaults,
therefore we should default to use settings from firmware.

Tested by: schakrabarti@microsoft.com
PR: 266248
MFC after: 1 week

(cherry picked from commit c243de11cf7c4bb3d67bbc1655b149037e5b04f1)