vdpa/octeon_ep: Fix error code in octep_process_mbox()

Return -EINVAL for invalid signatures.  Don't return success.

Fixes: 8b6c724c ("virtio: vdpa: vDPA driver for Marvell OCTEON DPU devices")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Message-Id: <623e885b-1a05-479e-ab97-01bcf10bf5b8@stanley.mountain>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>