irqchip/gic-v3-its: Don't override quirk settings with default values

When splitting the allocation of the ITS node from its configuration,
some of the default settings were kept in the latter instead of
being moved to the former.

This has the side effect of negating some of the quirk detections that
have happened in between, amongst which the dreaded Synquacer hack
(that also affect Dominic's TI platform).

Move the initialisation of these fields early, so that they can again be
overriden by the Synquacer quirk.

Fixes: 9585a495ac93 ("irqchip/gic-v3-its: Split allocation from initialisation of its_node")
Reported by: Dominic Rath <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Tested-by: Dominic Rath <[e-mail]>
Link: https://lore.kernel.org/r/20231024084831.GA3788@JADEVM-DRA
Link: https://lore.kernel.org/r/[e-mail]

drivers/irqchip/irq-gic-v3-its.c

···
 	}
 	its->cmd_base = (void *)page_address(page);
 	its->cmd_write = its->cmd_base;
-	its->get_msi_base = its_irq_get_msi_base;
-	its->msi_domain_flags = IRQ_DOMAIN_FLAG_ISOLATED_MSI;
 
 	err = its_alloc_tables(its);
 	if (err)
···
 	its->typer = gic_read_typer(its_base + GITS_TYPER);
 	its->base = its_base;
 	its->phys_base = res->start;
+	its->get_msi_base = its_irq_get_msi_base;
+	its->msi_domain_flags = IRQ_DOMAIN_FLAG_ISOLATED_MSI;
 
 	its->numa_node = numa_node;
 	its->fwnode_handle = handle;