xen/pci-swiotlb: reduce visibility of symbols

xen/pci-swiotlb: reduce visibility of symbols

xen_swiotlb and pci_xen_swiotlb_init() are only used within the file
defining them, so make them static and remove the stubs. Otoh
pci_xen_swiotlb_detect() has a use (as function pointer) from the main
pci-swiotlb.c file - convert its stub to a #define to NULL.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/aef5fc33-9c02-4df0-906a-5c813142e13c@suse.com
Signed-off-by: Juergen Gross <jgross@suse.com>