PCI/VFIO: use pcie_flags_reg instead of access PCI-E Capabilities Register

Currently, we use pcie_flags_reg to cache PCI-E Capabilities Register,
because PCI-E Capabilities Register bits are almost read-only. This patch
use pcie_caps_reg() instead of another access PCI-E Capabilities Register.
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/vfio_pci_config.c

@@ -1037,13 +1037,9 @@ static int vfio_cap_len(struct vfio_pci_device *vdev, u8 cap, u8 pos)
 		return byte;
 	case PCI_CAP_ID_EXP:
 		/* length based on version */
-		ret = pci_read_config_word(pdev, pos + PCI_EXP_FLAGS, &word);
-		if (ret)
-			return pcibios_err_to_errno(ret);
-
 		vdev->extended_caps = true;
 
-		if ((word & PCI_EXP_FLAGS_VERS) == 1)
+		if ((pcie_caps_reg(pdev) & PCI_EXP_FLAGS_VERS) == 1)
 			return PCI_CAP_EXP_ENDPOINT_SIZEOF_V1;
 		else
 			return PCI_CAP_EXP_ENDPOINT_SIZEOF_V2;