of/irq: improve error report on irq discovery process failure

On PowerPC machines some PCI slots might not have level triggered
interrupts capability (also know as level signaled interrupts),
leading of_irq_parse_pci() to complain by presenting error messages
on the kernel log - in this case, the properties "interrupt-map" and
"interrupt-map-mask" are not present on device's node in the device
tree.

This patch introduces a different message for this specific case,
and also reduces its level from error to warning. Besides, we warn
(once) that possibly some PCI slots on the system have no level
triggered interrupts available.
We changed some error return codes too on function of_irq_parse_raw()
in order other failure's cases can be presented in a more precise way.

Before this patch, when an adapter was plugged in a slot without level
interrupts capabilitiy on PowerPC, we saw a generic error message
like this:

    [54.239] pci 002d:70:00.0: of_irq_parse_pci() failed with rc=-22

Now, with this applied, we see the following specific message:

    [16.154] pci 0014:60:00.1: of_irq_parse_pci: no interrupt-map found,
    INTx interrupts not available

Finally, we standardize the error path in of_irq_parse_raw() by always
taking the fail path instead of returning directly from the loop.

Signed-off-by: Guilherme G. Piccoli <gpiccoli@linux.vnet.ibm.com>
Signed-off-by: Rob Herring <robh@kernel.org>