staging: comedi: amplc_pc236: check bus type in detach routine

When detaching the device in pc236_detach() mirror the bus type checks
performed by pc236_attach().  The existing tests are safe but rely on
dev->iobase being 0 when comedi_to_pci_dev(dev) is NULL.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>