include/linux/gtp.h: fix GTPA_MAX

Apply same fix as done by Pablo in linux.git:
> Subtract one to __GTPA_MAX, otherwise GTPA_MAX is off by 2

Change-Id: I42a358fc795d0e593e2c5509dcfa6ffdbad3cbfa
diff --git a/include/linux/gtp.h b/include/linux/gtp.h
index c525cb3..5d37b1c 100644
--- a/include/linux/gtp.h
+++ b/include/linux/gtp.h
@@ -28,6 +28,6 @@
 	GTPA_PAD,
 	__GTPA_MAX,
 };
-#define GTPA_MAX (__GTPA_MAX + 1)
+#define GTPA_MAX (__GTPA_MAX - 1)
 
 #endif /* _UAPI_LINUX_GTP_H_ */