PCI: endpoint: pci-epf-vntb: Fix sparse ntb->reg build warning

PCI: endpoint: pci-epf-vntb: Fix sparse ntb->reg build warning

  pci-epf-vntb.c:1128:33: sparse:     expected void [noderef] __iomem *base
  pci-epf-vntb.c:1128:33: sparse:     got struct epf_ntb_ctrl *reg

Add __iomem type cast in vntb_epf_peer_spad_read() and
vntb_epf_peer_spad_write().

Link: https://lore.kernel.org/r/20221102141014.1025893-8-Frank.Li@nxp.com
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Frank Li <frank.li@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Acked-by: Manivannan Sadhasivam <mani@kernel.org>