cxl: Fix sparse warnings

Make native_irq_wait() static and use NULL rather than 0 to initialise
phb->cfg_data in cxl_pci_vphb_add() to remove sparse warnings.

Signed-off-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Reviewed-by: Matthew R. Ochs <mrochs@linux.vnet.ibm.com>
Acked-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>