usbtmc: correct packet size bug

[scoudreau]

Code was only reading the first 64 bytes of a 512 bytes packet.

For some reason, code was working if receiving a single packet strictly below 512 bytes. But when sending a USBTMC message which lead to a data stage longer than a single packet, only the first 64 bytes where read.
This seems to be only due to the "arming" of transfer which where obviously only asking for 64 bytes (constant).

[hathach]

thank you, the main developer for usbtmc @pigrew only has FS device to test with. There is a recent update to get it running with HS #1531 , too bad, I keep "forgetting" to test it out. I will do a full revise later on.