swiotlb-xen: Convert to use macro

Rather than using static int max_dma_bits, this
can be coverted to use as macro.

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>