irqchip/gic-v3-its: Use allocation size from the prepare call

Now that .msi_prepare() gets called at the right time and not
with semi-random parameters, remove the ugly hack that tried
to fix up the number of allocated vectors.

It is now correct by construction.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250513163144.2215824-6-maz@kernel.org