MCP4728_VREF_MASK
outbuf[offset] = FIELD_PREP(MCP4728_VREF_MASK, ch->ref_mode);
outbuf[1] = FIELD_PREP(MCP4728_VREF_MASK, ch->ref_mode);
ch->ref_mode = FIELD_GET(MCP4728_VREF_MASK, r2);