PCI/MSI: Size device MSI domain with the maximum number of vectors

Zenghui reports that since 1396e89e ("genirq/msi: Move prepare() call
to per-device allocation"), his Multi-MSI capable device isn't working
anymore.

This is a consequence of 15c72f82 ("PCI/MSI: Add support for per device
MSI[X] domains"), which always creates a MSI domain of size 1, even in the
presence of Multi-MSI.

While this was somehow working until then, moving the .prepare() call ends
up sizing the ITS table with a tiny value for this device, and making the
endpoint driver unhappy.

Instead, always create the domain and call the .prepare() helper with the
maximum expected size.

Fixes: 1396e89e ("genirq/msi: Move prepare() call to per-device allocation")
Fixes: 15c72f82 ("PCI/MSI: Add support for per device MSI[X] domains")
Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Zenghui Yu <yuzenghui@huawei.com>
Reviewed-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Link: https://lore.kernel.org/all/20250603141801.915305-1-maz@kernel.org
Closes: https://lore.kernel.org/r/0b1d7aec-1eac-a9cd-502a-339e216e08a1@huawei.com