dist/tools/usb-serial: fix exception on missing entries

For some TTY interfaces no DB entry exists, which is reflected by
having a `None` in `tty[key]`. Trying to match a regex against `None`
in turn resulted then in an exception.

This fixes the issue by treating a filter applied on a non-existing
entry as not matching.

dist/tools/usb-serial/ttys.py

@@ -47,6 +47,9 @@ def filters_match(filters, tty):
     """
 
     for key, regex in filters:
+        if tty[key] is None:
+            return False
+
         if not regex.match(tty[key]):
             return False