usb: Do not try to print verbose trace when disabled

Right now if usb tracing is enabled but verbose is disabled
a debug assert will be triggered when ever a verbose trace point
is hit.  Instead of trying to print the NULL message, just return
early.

drivers/usbhost/usbhost_trace.c

@@ -202,7 +202,10 @@ void usbhost_trace1(uint16_t id, uint32_t u23)
   /* Get the format associated with the trace */
 
   fmt = usbhost_trformat1(id);
-  DEBUGASSERT(fmt);
+  if (fmt == NULL)
+    {
+      return;
+    }
 
   /* Just print the data using syslog() */
 
@@ -220,7 +223,10 @@ void usbhost_trace2(uint16_t id, uint8_t u7, uint16_t u16)
   /* Get the format associated with the trace */
 
   fmt = usbhost_trformat2(id);
-  DEBUGASSERT(fmt);
+  if (fmt == NULL)
+    {
+      return;
+    }
 
   /* Just print the data using syslog() */