gpio: remove explicit comparison with 0

No need to compare return value with 0. In case of non-zero
return value, the if condition will be true.

This makes intent a bit more clear to the reader.
"if (x) then", compared to "if (x is not zero) then".

Signed-off-by: Saiyam Doshi <saiyamdoshi.in@gmail.com>
Link: https://lore.kernel.org/r/20190907173910.GA9547@SD


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>