irqchip/gic-v4: Correctly deal with set_affinity on lazily-mapped VPEs

Zenghui points out that a recent change to the way set_affinity is
handled for VPEs has the potential to return an error if the VPE
hasn't been mapped yet (because the guest hasn't emited a MAPTI
command yet), affecting GICv4.0 implementations that rely on the
ITSList feature.

Fix this by making the set_affinity succeed in this case, and
return early, without trying to touch the HW.

Fixes: 1442ee00 ("irqchip/gic-v4: Don't allow a VMOVP on a dying VPE")
Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/all/20241027102220.1858558-1-maz@kernel.org
Link: https://lore.kernel.org/r/aab45cd3-e5ca-58cf-e081-e32a17f5b4e7@huawei.com