xen/sndfront: use xenbus_setup_ring() and xenbus_teardown_ring()

Simplify sndfront's ring creation and removal via xenbus_setup_ring()
and xenbus_teardown_ring().

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Tested-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com> # Arm64 only
Signed-off-by: Juergen Gross <jgross@suse.com>