Add advantest workarounds

Advantest (latterly ADCMT) implement a customised version of USB-TMC on their instruments. Specifically:
  * The maximum packet size is limited to 63 bytes
  * EOI is not signalled on reads
  * The instrument must be locked out with a vendor specific control transfer in order to execute commands (otherwise it just returns the most recent measurement)

This patch fixes most of these issues and works around the EOI by only accepting the first packet. Ideally I need to figure out what ADCMT's USB driver does in order to fix this "properly"...

I've also added a function which returns the "MyID" value for Advantest instruments. This is a user-programmable identification number which can be used to identify Advantest and ADCMT instruments.