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 d6203265, 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.