plat-nomadik: type secondary IRQ correctly

Coverity found that we were checking an unsigned variable for
>= zero. Type it correctly so that the check works as intended.

Signed-off-by: Virupax Sadashivpetimath <virupax.sadashivpetimath@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>