iface-modem: always ensure that current bands is a subset of supported bands

In QMI modems the logic behind supported and current bands is completely
separated in different services (DMS vs NAS). Actually, the list reported by NAS
as current band preferences seems to include more values than the ones reported
by DMS as supported bands (e.g. CDMA bands are reported even if the firmware
image is GSM/HSPA only).

So, just clean up the list of current preferred bands so that no more than those
given as supported is used.

src/mm-modem-helpers.h

@@ -61,6 +61,9 @@ gchar *mm_create_device_identifier (guint vid,
 
 guint mm_netmask_to_cidr (const gchar *netmask);
 
+GArray *mm_filter_current_bands (const GArray *supported_bands,
+                                 const GArray *current_bands);
+
 /*****************************************************************************/
 /* 3GPP specific helpers and utilities */
 /*****************************************************************************/