garbage collect always-true condition check

[jyoung8607]

GC condition check that never fails, correctly identified by an updated cppcheck (not a MISRA violation) in #2254.

/home/runner/work/panda/panda/board/drivers/usb.h:506:20: style: Condition 'resp_len!=-1' is always true [knownConditionTrueFalse]
      if (resp_len != -1) {
                   ^
/home/runner/work/panda/panda/board/drivers/usb.h:504:39: note: Calling function 'comms_control_handler' returns !<=-1
      resp_len = comms_control_handler(&control_req, response);
                                      ^
/home/runner/work/panda/panda/board/drivers/usb.h:506:20: note: Condition 'resp_len!=-1' is always true
      if (resp_len != -1) {

This check/case was added in #565, and is leftover after #1743.