Rename GSMTAP_CHANNEL_PDCH to GSMTAP_CHANNEL_PDTCH

PDCH is the physical channel, while actually we want to talk about
PDTCH the logical channel.  Introduce backwards compatibility define.

Change-Id: Id6782d2247846e2db29fc58d98684970a66db948
diff --git a/include/osmocom/core/gsmtap.h b/include/osmocom/core/gsmtap.h
index 1e19315..c226185 100644
--- a/include/osmocom/core/gsmtap.h
+++ b/include/osmocom/core/gsmtap.h
@@ -85,7 +85,9 @@
 #define GSMTAP_CHANNEL_TCH_H	0x0a
 #define GSMTAP_CHANNEL_PACCH	0x0b
 #define GSMTAP_CHANNEL_CBCH52	0x0c
-#define GSMTAP_CHANNEL_PDCH	0x0d
+#define GSMTAP_CHANNEL_PDTCH	0x0d
+/* for legacy reasons we use a mis-spelled name. PDCH is really the physical channel, but we use it as PDTCH */
+#define GSMTAP_CHANNEL_PDCH	GSMTAP_CHANNEL_PDTCH
 #define GSMTAP_CHANNEL_PTCCH	0x0e
 #define GSMTAP_CHANNEL_CBCH51	0x0f