temp/ofono: implement DTMF in qmimodem driver (MR 1922)

The TLVs are documented in GobiAPI. I pass 0xff for the call ID, as the
stock RIL appears to always do. I would guess it means "current foreground
call."

The call ID is returned in TLV 0x10, but I didn't implement parsing of
that.

Co-authored-by: Joey Hewitt <joey@joeyhewitt.com>
Acked-by: Alexey Andreyev <aa13q@ya.ru>