irqchip/ti-sci-intr: Fix unsigned comparison to zero

ti_sci_intr_xlate_irq() return -ENOENT on fail, p_hwirq
should be int type.

Fixes: a5b659bd ("irqchip/ti-sci-intr: Add support for INTR being a parent to INTR")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20200826035321.18620-1-yuehaibing@huawei.com