powerpc/pseries: Use of_irq_get helper() in request_event_sources_irqs()

Instead of calling both of_irq_parse_one() and
irq_create_of_mapping(), call of_irq_get() instead which does
essentially the same thing. of_irq_get() also calls irq_find_host()
for deferred probe support, but this should be fine as
irq_create_of_mapping() also calls that internally. This gets us
closer to making the former 2 functions static.

In the process of simplifying request_event_sources_irqs(), combine
the the pr_err() and WARN_ON() calls to just a WARN().

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>