USB: serial: reuse generic write urb and bulk-out buffer

Reuse first write urb and bulk-out buffer of the generic write
implementation for drivers that rely on port->write_urb rather than
allocating them separately.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>