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/pcie_layerscape_gen4.c b/drivers/pci/pcie_layerscape_gen4.c
index 1fd8761..cec61fa 100644
--- a/drivers/pci/pcie_layerscape_gen4.c
+++ b/drivers/pci/pcie_layerscape_gen4.c
@@ -227,7 +227,7 @@
 	return pcie->cfg + offset;
 }
 
-static int ls_pcie_g4_read_config(struct udevice *bus, pci_dev_t bdf,
+static int ls_pcie_g4_read_config(const struct udevice *bus, pci_dev_t bdf,
 				  uint offset, ulong *valuep,
 				  enum pci_size_t size)
 {