PCI: keystone: 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>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-By: Murali Karicheri <m-karicheri2@ti.com>