If we failed to probe/attach somehow, we never have a sc->sc_hcca

If we failed to probe/attach somehow, we never have a sc->sc_hcca
but the present PCI probe code still thinks we are there as the pci attach
can't return an error code.

This means we are in the shared interrupt list, but have not been set up.
If we are sharing ints with another device, ohci_intr will be called and will
coredump on a NULL reference. So just return if it is called when not set up.

This fixes the symptom and not the cause.
The right answer is to let the PCI system know that the attach failed,
or to fail earlier (in the PCI probe).
The attach() is a void fn() so it can't return failure..