PCI: pci-bridge-emul: Make expansion ROM Base Address register read-only

If expansion ROM is unsupported (which is the case of pci-bridge-emul.c
driver) then ROM Base Address register must be implemented as read-only
register that return 0 when read, same as for unused Base Address
registers.

Link: https://lore.kernel.org/r/20211124155944.1290-2-pali@kernel.org
Fixes: 23a5fba4d9 ("PCI: Introduce PCI bridge emulated config space common logic")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: stable@vger.kernel.org

drivers/pci/pci-bridge-emul.c

@@ -139,8 +139,13 @@ struct pci_bridge_reg_behavior pci_regs_behavior[PCI_STD_HEADER_SIZEOF / 4] = {
 		.ro = GENMASK(7, 0),
 	},
 
+	/*
+	 * If expansion ROM is unsupported then ROM Base Address register must
+	 * be implemented as read-only register that return 0 when read, same
+	 * as for unused Base Address registers.
+	 */
 	[PCI_ROM_ADDRESS1 / 4] = {
-		.rw = GENMASK(31, 11) | BIT(0),
+		.ro = ~0,
 	},
 
 	/*