ehea: fix losing of NEQ events when one event occurred early

The NEQ interrupt is only triggered when there was no previous pending
interrupt. If we request irq handling after an interrupt has occurred,
we will never get an interrupt until we call H_RESET_EVENTS.

Events seem to be cleared when we first register the NEQ. So, when we
requested irq handling right after registering it, a possible race with
an interrupt was much less likely. Now, there is a chance we may lose
this race and never get any events.

The fix here is to poll and acknowledge any events that might have
happened right after registering the irq handler.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>