GSM/GPRS 04.08: Don't print numeric mobile identity types

I guess none of our users knows what a mi_type=0x02 is, but most would
know what an IMSI or a TMSI is.  So let's use the newly introduced
gsm48_mi_type_name() function to fix this.
diff --git a/openbsc/src/osmo-bsc/osmo_bsc_filter.c b/openbsc/src/osmo-bsc/osmo_bsc_filter.c
index 6058d7e..e975733 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_filter.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_filter.c
@@ -73,8 +73,8 @@
 
 	gsm48_paging_extract_mi(resp, msgb_l3len(msg) - sizeof(*gh),
 				mi_string, &mi_type);
-	DEBUGP(DRR, "PAGING RESPONSE: mi_type=0x%02x MI(%s)\n",
-		mi_type, mi_string);
+	DEBUGP(DRR, "PAGING RESPONSE: MI(%s)=%s\n",
+		gsm48_mi_type_name(mi_type), mi_string);
 
 	switch (mi_type) {
 	case GSM_MI_TYPE_TMSI: