BaseTools/Source/C/GenSec: Fix EFI_SECTION_FREEFORM_SUBTYPE_GUID header

BaseTools/Source/C/GenSec: Fix EFI_SECTION_FREEFORM_SUBTYPE_GUID header

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4021

When the size of a EFI_SECTION_FREEFORM_SUBTYPE_GUID section required
the use of EFI_FREEFORM_SUBTYPE_GUID_SECTION2 header, set the section
type to EFI_SECTION_FREEFORM_SUBTYPE_GUID.

Cc: Leif Lindholm <llindhol@qti.qualcomm.com>
Cc: Andrew Fish <afish@apple.com>
Cc: Konstantin Aladyshev <aladyshev22@gmail.com>
Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Acked-by: Konstantin Aladyshev <aladyshev22@gmail.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Leif Lindholm <llindhol@qti.qualcomm.com>