gpio: xgene: fix type of variable containing error codes

ret variable can contain error values and is compared with zero.
Its type must be signed.

The problem has been detected using coccinelle script
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>