irqchip/gic-v3: Fix ICC_SGI1R_EL1.INTID decoding mask

The INTID mask is wrong, and is made a signed value, which has
nteresting effects in the KVM emulation. Let's sanitize it.

Cc: stable@vger.kernel.org
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>