virtio_pci(4): Fix typo in read_ivar method

Prior to this revision, vtpci's BUS_READ_IVAR method on VIRTIO_IVAR_SUBVENDOR
accidentally returned the PCI subdevice.

The typo seems to have been introduced with the original commit adding
VIRTIO_IVAR_{{SUB,}DEVICE,{SUB,}VENDOR} to virtio_pci. The commit log and code
strongly suggest that the ivar was intended to return the subvendor rather than
the subdevice; it was likely just a copy/paste mistake.

Go ahead and rectify that.