Input: gpio_keys - don't report events on gpio failure

In the cases where the gpio chip fails to acquire the current state an
error is reported back to gpio_keys. This is currently interpreted as if
the line went high, which just confuses the developer.

This patch introduces an error print in this case and skipps the
reporting of a input event; to aid in debugging this issue.

Reported-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>