irqchip/irq-ls1x: Missing error code in ls1x_intc_of_init()

Currently, when irq_domain_add_linear() fails, the error code does not get
set so it returns zero which is wrong.  Fix it by setting the appropriate
error code.

Fixes: 9e543e22 ("irqchip: Add driver for Loongson-1 interrupt controller")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: kernel-janitors@vger.kernel.org
Link: https://lkml.kernel.org/r/20190329062136.GQ32613@kadam