swiotlb-xen: add struct device * parameter to xen_phys_to_bus

No functional changes. The parameter is unused in this patch but will be
used by next patches.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Tested-by: Corey Minyard <cminyard@mvista.com>
Tested-by: Roman Shaposhnik <roman@zededa.com>
Link: https://lore.kernel.org/r/20200710223427.6897-3-sstabellini@kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>