broadband-modem-huawei: check for ^CVOICE support and enable audio streaming

USB sticks only support voice if ^CVOICE returns 0.  And to enable audio
streaming on the "Application" port (whatever is returned by AT^DDSETEX=?)
we need to send AT^DDSETEX=<port> after starting the call.  After that
the serial port will send and accept signed 16-bit 8000hz PCM audio, or
whatever format is returned by ^CVOICE?.