gbproxy: Replace 'mi_data' by 'imsi'

Since at all places where mi_data/mi_data_len is used it will always
contain an IMSI. Thus the names of the identifiers have been updated
accordingly for clarity.

Sponsored-by: On-Waves ehf
diff --git a/openbsc/src/gprs/gb_proxy.c b/openbsc/src/gprs/gb_proxy.c
index dfaa380..2dfb4d8 100644
--- a/openbsc/src/gprs/gb_proxy.c
+++ b/openbsc/src/gprs/gb_proxy.c
@@ -362,7 +362,7 @@
 	if (!tlli_info)
 		return 1;
 
-	if (!tlli_info->imsi_acq_pending && tlli_info->mi_data_len > 0)
+	if (!tlli_info->imsi_acq_pending && tlli_info->imsi_len > 0)
 		return 1;
 
 	if (parse_ctx->g48_hdr &&
@@ -376,7 +376,7 @@
 		}
 	}
 
-	if (tlli_info->imsi_acq_pending && tlli_info->mi_data_len > 0) {
+	if (tlli_info->imsi_acq_pending && tlli_info->imsi_len > 0) {
 		int is_ident_resp =
 			parse_ctx->g48_hdr &&
 			parse_ctx->g48_hdr->proto_discr == GSM48_PDISC_MM_GPRS &&