powerpc/eeh-powernv: Fix unbalanced IRQ warning

pnv_eeh_next_error() re-enables the eeh opal event interrupt but it
gets called from a loop if there are more outstanding events to
process, resulting in a warning due to enabling an already enabled
interrupt. Instead the interrupt should only be re-enabled once the
last outstanding event has been processed.

Tested-by: Daniel Axtens <dja@axtens.net>
Reported-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Alistair Popple <alistair@popple.id.au>
Acked-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>