ptp: Return -EFAULT on copy_to_user() errors

copy_to_user() returns the number of bytes remaining, but we want a
negative error code in ptp_ioctl.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: John Stultz <john.stultz@linaro.org>