gsm_utils: add function to get value strings for gsm48_chan_mode

The enum gsm48_chan_mode has value strings, but no function to retrieve
those strings. Lets add one.

Change-Id: If715705e1ebdfcfdfae1c2099932f7bb8b3861a7
diff --git a/include/osmocom/gsm/gsm_utils.h b/include/osmocom/gsm/gsm_utils.h
index 6ff4459..7eda5b9 100644
--- a/include/osmocom/gsm/gsm_utils.h
+++ b/include/osmocom/gsm/gsm_utils.h
@@ -228,6 +228,8 @@
 };
 
 extern const struct value_string gsm_chan_t_names[];
+static inline const char *gsm_chan_t_name(enum gsm48_chan_mode val)
+{ return get_value_string(gsm_chan_t_names, val); }
 
 /* Deprectated functions */
 /* Limit encoding and decoding to use no more than this amount of buffer bytes */