scpi-dmm: run OPC queries immediately before essential commands

scpi-dmm: run OPC queries immediately before essential commands

The current implementation of the SCPI DMM driver is conservative about
checking the device's being operational, but the *OPC? queries are found
in unfortunate locations. Run the OPC query right before running the
next "actual" command, not afterwards. And certainly not between sending
requests and potentially gathering responses in subsequent calls.

This commit does not change current behaviour, but improves maintenance
before pending commits.