irqchip/gic-v3-its: Fix VPE activate callback return value

its_vpe_irq_domain_activate should always return 0. Really. There
is not a single case why it wouldn't.  So this "return true;" is
really a copy/paste issue that got revealed now that we actually
check the return value of the activate method.

Brown paper bag day.

Fixes: 2247e1bf ("irqchip/gic-v3-its: Limit scope of VPE mapping to be per ITS")
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>