gpio: Remove impossible checks on container_of() result

container_of() does pointer math on the pointer that's passed in.
If it were to return a NULL pointer the value passed in would
need to be perfectly offset from 0 to make that so. Remove these
checks because they don't make sense.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>