[PA-RISC] Fix must_check warnings in drivers.c

Panic if we can't register the parisc bus or the root parisc device.
There's no way we can boot without them, so let the user know ASAP.

If we can't register a parisc device, handle the failure gracefully.

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