Fix unbalanced USBDevice unlock

Remove calls to unlock which are not preceded by calls to lock.
Also move the location of the unlock underflow assert so unbalanced
unlocking during post processing is caught.

usb/device/USBDevice/USBDevice.cpp

@@ -331,13 +331,11 @@ void USBDevice::_complete_request_xfer_done()
     _transfer.user_callback = None;
     if (_abort_control) {
         _control_abort();
-        unlock();
         return;
     }
 
     if (!success) {
         _phy->ep0_stall();
-        unlock();
         return;
     }
 
@@ -405,7 +403,6 @@ void USBDevice::_complete_set_configuration()
     _transfer.user_callback = None;
     if (_abort_control) {
         _control_abort();
-        unlock();
         return;
     }
 
@@ -478,7 +475,6 @@ void USBDevice::_complete_set_interface()
     _transfer.user_callback = None;
     if (_abort_control) {
         _control_abort();
-        unlock();
         return;
     }
 
@@ -716,7 +712,6 @@ void USBDevice::_complete_request()
         } else {
             _control_abort();
         }
-        unlock();
         return;
     }
 
@@ -1563,8 +1558,6 @@ void USBDevice::lock()
 
 void USBDevice::unlock()
 {
-    MBED_ASSERT(_locked > 0);
-
     if (_locked == 1) {
         // Perform post processing before fully unlocking
         while (_post_process != NULL) {
@@ -1574,6 +1567,7 @@ void USBDevice::unlock()
         }
     }
 
+    MBED_ASSERT(_locked > 0);
     _locked--;
     core_util_critical_section_exit();
 }