gpio: Propagate errors from chip->get()

It's possible to have gpio chips hanging off unreliable remote buses
where the get() operation will fail to acquire a readout of the current
gpio state. Propagate these errors to the consumer so that they can
act on, retry or ignore these failing reads, instead of treating them as
the line being held high.

Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>