irqchip/irq-mvebu-icu: Fix wrong private data retrieval

The irq_domain structure has an host_data pointer that just stores
private data. It is meant to not be touched by the IRQ core. However,
when it comes to MSI, the MSI layer adds its own private data there
with a structure that also has a host_data pointer.

Because this IRQ domain is an MSI domain, to access private data we
should do a d->host_data->host_data, also wrapped as
'platform_msi_get_host_data()'.

This bug was lying there silently because the 'icu' structure retrieved
this way was just called by dev_err(), only producing a
'(NULL device *):' output on the console.

Reviewed-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>