drm/i915/gvt: Fix an error code in ppgtt_populate_spt_by_guest_entry()

"ret" is uninitialized on this path but it should be -EINVAL.

Fixes: 930c8dfe ("drm/i915/gvt: Check if get_next_pt_type() always returns a valid value")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>