irqchip/gic-v3-its: Fix typo in a comment in its_msi_prepare()

The word 'entirely' has been misspelt in a comment in its_msi_prepare().

Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>