gpio: tz1090: Fix error handling of irq_of_parse_and_map

irq_of_parse_and_map() returns 0 on error, so testing for negative
result never works.

Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Acked-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>