irqchip/ti-sci-inta: Fix kernel crash if irq_create_fwspec_mapping fail

irq_create_fwspec_mapping() can fail, returning 0 as parent_virq. In this
case vint_desc is going to be NULL in ti_sci_inta_alloc_irq() which will
cause NULL pointer dereference.

Also note that irq_create_fwspec_mapping() returns 'unsigned int' so the
check '<=' was wrong.

Use -EINVAL if irq_create_fwspec_mapping() returned with 0.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>