virtio_pmem: fix sparse warning

This patch fixes below sparse warning related to __virtio
type in virtio pmem driver. This is reported by Intel test
bot on linux-next tree.

nd_virtio.c:56:28: warning: incorrect type in assignment
                                (different base types)
nd_virtio.c:56:28:    expected unsigned int [unsigned] [usertype] type
nd_virtio.c:56:28:    got restricted __virtio32
nd_virtio.c:93:59: warning: incorrect type in argument 2
                                (different base types)
nd_virtio.c:93:59:    expected restricted __virtio32 [usertype] val
nd_virtio.c:93:59:    got unsigned int [unsigned] [usertype] ret

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>