gpio: mockup: return -EFAULT if copy_from_user() fails

copy_from_user() returns the number of bytes remaining to be copied but
we want to return negative error codes on failue.

Fixes: 9202ba23 ("gpio: mockup: implement event injecting over debugfs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>