broadmobi: add MM_PLUGIN_REQUIRED_QCDM flag

This fixes an issue where, with an BM818 connected over USB,
suspending/resuming the modem causes the modem to reset immediately.

The culprit was bisected to d6203265c2, where QCDM probing was disabled
unless explicitly requested, so I assume this modem needs it based on
the constant resetting when power management was enabled on the modem's
usb connection.

plugins/broadmobi/mm-plugin-broadmobi.c

@@ -76,7 +76,7 @@ mm_plugin_create (void)
                       MM_PLUGIN_ALLOWED_SUBSYSTEMS, subsystems,
                       MM_PLUGIN_ALLOWED_VENDOR_IDS, vendor_ids,
                       MM_PLUGIN_ALLOWED_AT,         TRUE,
-                      MM_PLUGIN_ALLOWED_QCDM,       TRUE,
+                      MM_PLUGIN_REQUIRED_QCDM,      TRUE,
                       MM_PLUGIN_ALLOWED_QMI,        TRUE,
                       NULL));
 }