virtio_config: fix up warnings on parisc

Apparently, on parisc le16_to_cpu returns an int. virtio_cread_le
is very strict about type sizes so it causes a warning.
Fix it up by casting to the correct type.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/r/20200805235550.1451637-1-mst@redhat.com