virtio_console: fix uapi header

commit 6407d75afd08545f2252bb39806ffd3f10c7faac upstream.

uapi should use __u32 not u32.
Fix a macro in virtio_console.h which uses u32.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>