swiotlb-xen: drop leftover __ref

Commit a98f5654 ("xen-swiotlb: split xen_swiotlb_init") should not
only have added __init to the split off function, but also should have
dropped __ref from the one left.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

Link: https://lore.kernel.org/r/7cd163e1-fe13-270b-384c-2708e8273d34@suse.com


Signed-off-by: Juergen Gross <jgross@suse.com>