gpio: No need to cast away return value of debugfs_create_file()

It is fine to ignore the return value (and encouraged), so need to cast
away the return value, you will not get a build warning at all.

Cc: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc: linux-gpio@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>