drm/amdkfd: Fix error code sign for EINVAL in svm_ioctl()

Use negative error code -EINVAL instead of positive EINVAL in the default
case of svm_ioctl() to conform to Linux kernel error code conventions.

Fixes: 42de677f ("drm/amdkfd: register svm range")
Signed-off-by: Qianfeng Rong <rongqianfeng@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>