gpio: pca953x: initialize ret to zero to avoid returning garbage

ret is not initialized so it contains garbage.  Ensure garbage
is not returned in the case that pdata && pdata->teardown is false
by initializing ret to 0.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>