irqchip/mbigen: Checking for IS_ERR() instead of NULL

of_platform_device_create() returns NULL on error, it never returns
error pointers.

Fixes: ed2a1002 ('irqchip/mbigen: Handle multiple device nodes in a mbigen module')
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>