USB: serial: remove unnecessary bulk-urb re-fills

Remove unnecessary re-fills of bulk urbs whose fields have not changed
since port probe.

Compile-only tested.

Cc: Matthias Bruestle and Harald Welte <support@reiner-sct.com>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>