gsm_08_08.h: Add constants for Channel Type in signalling mode

This is from TS 08.08 3.2.2.11 directly. The choices for Data mode
and Speech mode were already present, but not for Signalling mode

Change-Id: I9e24841ea877a9a78dc4d2bd14cbf60c4bea79a6
Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/include/osmocom/gsm/protocol/gsm_08_08.h b/include/osmocom/gsm/protocol/gsm_08_08.h
index 721da33..67dc6ee 100644
--- a/include/osmocom/gsm/protocol/gsm_08_08.h
+++ b/include/osmocom/gsm/protocol/gsm_08_08.h
@@ -437,6 +437,20 @@
 	GSM0808_SPEECH_PERM_NO_CHANGE = 0x1f,
 };
 
+/* GSM 08.08 3.2.2.11 Channel Type */
+enum gsm0808_chan_rate_type_sign {
+	GSM0808_SIGN_ANY                 = 0x00,
+	GSM0808_SIGN_SDCCH               = 0x01,
+	GSM0808_SIGN_SDCCH_FULL_BM       = 0x02,
+	GSM0808_SIGN_SDCCH_HALF_LM       = 0x03,
+	GSM0808_SIGN_FULL_BM             = 0x08,
+	GSM0808_SIGN_HALF_LM             = 0x09,
+	GSM0808_SIGN_FULL_PREF           = 0x0a,
+	GSM0808_SIGN_HALF_PREF           = 0x0b,
+	GSM0808_SIGN_FULL_PREF_NO_CHANGE = 0x1a,
+	GSM0808_SIGN_HALF_PREF_NO_CHANGE = 0x1b,
+};
+
 /*! GSM 08.08 3.2.2.11 Channel Type
  *  (see also 3GPP TS 48.008, section 3.2.2.11) */
 enum gsm0808_permitted_speech {