[PARISC] Return PDC_OK when alloc_pa_dev fails to enumerate all devices

Return PDC_OK when device registration fails so that we enumerate all
subsequent devices, even when we get two devices with the same hardware
path (which should never happen, but does with at least one revision of
rp8400 firmware).

Signed-off-by: Matthew Wilcox <willy@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>