Add support for Advantest/ADCMT version of USBTMC

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"...
Merge pull request #5 from philpem/add_advantest_workarounds

Add advantest workarounds