irqchip/xilinx: Fill error code when irq domain registration fails

There is no ret filled in case of irq_domain_add_linear() failure.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Stefan Asserhall <stefan.asserhall@xilinx.com>
Link: https://lore.kernel.org/r/20200317125600.15913-3-mubin.usman.sayyed@xilinx.com