broadband-modem: detect old Mediatek 62xx chipsets as GSM-capable

They don't necessarily reply to +WS46 but like old Motorola ESX
devices they reply to +CGMM with a known string.

Fixes: https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/980

Signed-off-by: Dan Williams <dan@ioncontrol.co>
(cherry picked from commit 8cf25a97a1)

src/mm-broadband-modem.c

@@ -768,11 +768,14 @@ parse_caps_cgmm (MMBaseModem   *self,
     if (!response)
         return MM_BASE_MODEM_AT_RESPONSE_PROCESSOR_RESULT_CONTINUE;
 
-    /* This check detects some really old Motorola GPRS dongles and phones */
+    /* This check detects some really old Motorola and Mediatek GPRS dongles
+     * and phones.
+     */
     if (strstr (response, "GSM900") ||
         strstr (response, "GSM1800") ||
         strstr (response, "GSM1900") ||
-        strstr (response, "GSM850")) {
+        strstr (response, "GSM850") ||
+        strstr (response, "MTK2")) {
         /* At least, it's a GSM modem */
         *result = g_variant_new_uint32 (MM_MODEM_CAPABILITY_GSM_UMTS);
         return MM_BASE_MODEM_AT_RESPONSE_PROCESSOR_RESULT_SUCCESS;