irqchip/gic: WARN if setting the interrupt type for a PPI fails

Setting the interrupt type for private peripheral interrupts (PPIs) may
not be supported by a given GIC because it is IMPLEMENTATION DEFINED
whether this is allowed. There is no way to know if setting the type is
supported for a given GIC and so the value written is read back to
verify it matches the desired configuration. If it does not match then
an error is return.

There are cases where the interrupt configuration read from firmware
(such as a device-tree blob), has been incorrect and hence
gic_configure_irq() has returned an error. This error has gone
undetected because the error code returned was ignored but the interrupt
still worked fine because the configuration for the interrupt could not
be overwritten.

Given that this has done undetected and that failing to set the
configuration for a PPI may not be a catastrophic, don't return an error
but WARN if we fail to configure a PPI. This will allows us to fix up
any places in the kernel where we should be checking the return status
and maintain backward compatibility with firmware images that may have
incorrect PPI configurations.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>