This should be called by a parallel port device driver that
has registered itself using parport_register_driver when it
is about to be unloaded.
When it returns, the driver's attach routine will no longer
be called, and for each port that attach was called for, the
detach routine will have been called.
If the caller's attach function can block, it is their
responsibility to make sure to wait for it to exit before
unloading.
All the driver's detach calls are guaranteed to have
finished by the time this function returns.
The driver's detach call is not allowed to block.