[gsm48] Move parsing of the MI from to gsm_04_08_utils.c

The parsing of the IMSI is needed for the MSC part as well. Move
it to the gsm_04_08_utils.c so it can be used.
diff --git a/openbsc/include/openbsc/gsm_04_08.h b/openbsc/include/openbsc/gsm_04_08.h
index 2518dfd..8f0e465 100644
--- a/openbsc/include/openbsc/gsm_04_08.h
+++ b/openbsc/include/openbsc/gsm_04_08.h
@@ -684,6 +684,7 @@
 
 #define GSM48_TMSI_LEN	5
 #define GSM48_MID_TMSI_LEN	(GSM48_TMSI_LEN + 2)
+#define GSM48_MI_SIZE 32
 
 
 struct msgb;
@@ -708,6 +709,7 @@
 struct msgb *gsm48_msgb_alloc(void);
 int gsm48_sendmsg(struct msgb *msg, struct gsm_trans *trans);
 int generate_mid_from_tmsi(u_int8_t *buf, u_int32_t tmsi);
+int gsm48_mi_to_string(char *string, const int str_len, const u_int8_t *mi, const int mi_len);
 
 int gsm48_send_rr_release(struct gsm_lchan *lchan);
 int gsm48_send_rr_app_info(struct gsm_lchan *lchan, u_int8_t apdu_id,