Fixes: 8034f715 ("powernv/opal-dump: Convert to irq domain")

Converts all the return explicit number to a more proper IRQ_HANDLED,
which looks proper incase of interrupt handler returning case.

Here, It also removes error message like "nobody cared" which was
getting unveiled while returning -1 or 0 from handler.
Signed-off-by: Mukesh Ojha <mukesh02@linux.vnet.ibm.com>
Reviewed-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>