gpiolib: remove unneeded assignation

ret is assigned value from of_property_read_string_index but is
overwritten before being used so remove it.
Also fix coverity CID 1309759

Reported-by: coverity (CID 1309759)
Signed-off-by: Laurent Navet <laurent.navet@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>