drm/amdgpu: debugfs: fix error codes in write functions

There are two error code bugs here.  The copy_to/from_user() functions
return the number of bytes remaining (a positive number).  We should
return -EFAULT if the copy fails.

Second if we fail because "context.resp_status" is non-zero then return
-EINVAL instead of zero.

Fixes: e50d9ba0 ("drm/amdgpu: Add debugfs TA load/unload/invoke support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>