Revert "gpio: revert get() to non-errorprogating behaviour"

This reverts commit 45ad7db9.

We have fixed all the drivers that were returning ambious values
not clamped to [0,1] or an error code, so return the error
propagating behaviour of the API.

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