OvmfPkg/OvmfPkg*.dsc: Change PCDs type about status code

OvmfPkg/OvmfPkg*.dsc: Change PCDs type about status code

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2786

Since the type of PcdStatusCodeUseSerial and PcdStatusCodeUseMemory
in MdeModulePkg.dec are changed, so change them from
PcdsFeatureFlag to PcdsFixedAtBuild in dsc files.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Signed-off-by: Ming Tan <ming.tan@intel.com>