ClevoOpenBoardPkg/N1xxWU: Enable continous mode Serial IRQ

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2068



This change sets the serial IRQ mode to continuous to allow the PS/2
keyboard and mouse to function properly in the OS. In the UEFI boot
environment, the device uses a polling model so this change is not
strictly required for that environment.

Cc: Ankit Sinha <ankit.sinha@intel.com>
Cc: Nate DeSimone <nathaniel.l.desimone@intel.com>
Signed-off-by: Michael Kubacki <michael.a.kubacki@intel.com>
Reviewed-by: Nate DeSimone <nathaniel.l.desimone@intel.com>