nios2/uaccess: fix sparse errors

virtio wants to read bitwise types from userspace using get_user.  At the
moment this triggers sparse errors, since the value is passed through an
integer.

Fix that up using __force.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Ley Foon Tan <lftan@altera.com>