drm/msm: fix an error code in the ioctl

The copy_to/from_user() functions return the number of bytes remaining
to be copied but we should return -EFAULT to the user.

Fixes: f05c83e7 ("drm/msm: add uapi to get/set debug name")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rob Clark <robdclark@gmail.com>