huawei: implement ussd encoding/decoding

Huawei wants the USSD as packed GSM.

src/mm-generic-gsm.c

@@ -4820,9 +4820,9 @@ ussd_encode (MMModemGsmUssd *modem, const char* command, guint *scheme)
     if (!success)
         goto out;
 
+    *scheme = MM_MODEM_GSM_USSD_SCHEME_7BIT;
     /* convert to hex representation */
     hex = utils_bin2hexstr (ussd_command->data, ussd_command->len);
-    *scheme = 15;
 
  out:
     g_byte_array_free (ussd_command, TRUE);

src/mm-modem-gsm-ussd.h

@@ -80,6 +80,10 @@ void mm_modem_gsm_ussd_cancel (MMModemGsmUssd *self,
                                MMModemFn callback,
                                gpointer user_data);
 
+/* CBS data coding scheme - 3GPP TS 23.038 */
+#define MM_MODEM_GSM_USSD_SCHEME_7BIT 0b00001111;
+#define MM_MODEM_GSM_USSD_SCHEME_UCS2 0b01001000;
+
 char *mm_modem_gsm_ussd_encode (MMModemGsmUssd *self,
                                 const char* command,
                                 guint *scheme);