gpio: em: Fix checking return value of irq_alloc_descs

irq_alloc_descs() returns negative error code on failure.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>