At the moment if request_event_sources_irqs() can't allocate or request
the interrupt, it just does a KERN_ERR printk. This may be fine for the
existing RAS code where if we miss an EPOW event it just means that the
event won't be logged and if we miss one of the RAS errors then we could
miss an event that we perhaps should take action on.

But, for the upcoming IO events code that will use event-sources if we
can't allocate or request the interrupt it means we'd potentially miss
an interrupt from the device. So, let's add a WARN_ON() in this error
case so that we're a bit more vocal when something's amiss.

While we're at it, also use pr_err() to neaten the code up a bit.
Signed-off-by: Mark Nelson <markn@au1.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>