vdpa: fix typos in the comments for __vdpa_alloc_device()

Fix two typos in the comments for __vdpa_alloc_device().

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200527060528.9100-1-jasowang@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>