USB-serial drivers must not be unbound from their ports before the
corresponding USB driver is unbound from the parent interface so
suppress the bind and unbind attributes.
Unbinding a serial driver while it's port is open is a sure way to
trigger a crash as any driver state is released on unbind while port
hangup is handled on the parent USB interface level. Drivers for
multiport devices where ports share a resource such as an interrupt
endpoint also generally cannot handle individual ports going away.