[os2] Capitalize table tag and struct

Other tables follow the case of the OT tag, except this one.
diff --git a/src/hb-subset.cc b/src/hb-subset.cc
index ed61c62..a1c52c5 100644
--- a/src/hb-subset.cc
+++ b/src/hb-subset.cc
@@ -171,8 +171,8 @@
     case HB_OT_TAG_cmap:
       result = _subset<const OT::cmap> (plan);
       break;
-    case HB_OT_TAG_os2:
-      result = _subset<const OT::os2> (plan);
+    case HB_OT_TAG_OS2:
+      result = _subset<const OT::OS2> (plan);
       break;
     case HB_OT_TAG_post:
       result = _subset<const OT::post> (plan);