irqchip/imx-irqsteer: Fix of_property_read_u32() error handling

gcc points out that irqs_num is not initialized when of_property_read_u32()
is an empty stub function:

                 Included from drivers/irqchip/irq-imx-irqsteer.c:7:
drivers/irqchip/irq-imx-irqsteer.c: In function 'imx_irqsteer_probe':
include/uapi/linux/kernel.h:13:49: error: 'irqs_num' may be used uninitialized in this function [-Werror=maybe-uninitialized]

The same can actually happen with CONFIG_OF=y as well, though we don't
get a warning then.

Add error checking here that lets the code deal with missing or
invalid properties as well as avoid the warning.

Fixes: 28528fca ("irqchip/imx-irqsteer: Add multi output interrupts support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>