scpi-dmm: move OPC availability check to after IDN device identification

scpi-dmm: move OPC availability check to after IDN device identification

Assume that *IDN? requests don't involve expensive processing, and thus
don't require *OPC? before their execution. This makes scpi-dmm follow
the good tradition to only use non-standard or model dependent features
after the device was successfully identified. Which avoids confusion on
unrelated devices.

Exclusively check for *OPC? availability on Owon devices, because these
motivated the introduction of the test in the first place. This avoids
confusion on other devices where short timeouts result in errors because
the response is not retrieved although the command is supported (observed
on Agilent meters connected via USBTMC).