dm: pci: Update the PCI read_config() method to const dev *

At present this method uses a non-const udevice pointer, but the call
should not modify the device. Use a const pointer.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/pci/pci-aardvark.c b/drivers/pci/pci-aardvark.c
index aa0b4bc..f1527ac 100644
--- a/drivers/pci/pci-aardvark.c
+++ b/drivers/pci/pci-aardvark.c
@@ -297,7 +297,7 @@
  *
  * Return: 0 on success
  */
-static int pcie_advk_read_config(struct udevice *bus, pci_dev_t bdf,
+static int pcie_advk_read_config(const struct udevice *bus, pci_dev_t bdf,
 				 uint offset, ulong *valuep,
 				 enum pci_size_t size)
 {