Don't read fixed size of 1024

[matthewrankin]

In device.go the Read(p []byte) method uses a fixed length of 1024 to read the USBTMC bulk-in endpoint. Seems like I should refactor this.

[matthewrankin]

The magic number that needs to be eliminated is no longer 1024. It is now 512 in func (d *Device) readRemoveHeader(p []byte) (n int, err error).