irqchip: brcmstb-l2: Fix error handling of irq_of_parse_and_map

Return value of irq_of_parse_and_map() is unsigned int, with 0
indicating failure, so testing for negative result never works.

Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Kevin Cernekee <cernekee@gmail.com>
Link: https://lkml.kernel.org/r/20141114221642.GA37468@dtor-ws
Signed-off-by: Jason Cooper <jason@lakedaemon.net>