stlink_open_usb: load device param returning -1 is not an usb issue.

do not abort in case of load device param "error" :  this is supposed to return -1
if device is "unknown" or not detected, which is not reaaally an error.

we want probe to tell us what is the connected device even if we can not operate it
we want probe to tell us that it can not operate it, not to hide the connected stlink