firewire: qualify config ROM cache pointers as const pointers

Several config ROM related functions only peek at the ROM cache; mark
their arguments as const pointers.  Ditto fw_device.config_rom and
fw_unit.directory, as the memory behind them is meant to be write-once.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
diff --git a/drivers/firewire/sbp2.c b/drivers/firewire/sbp2.c
index d485cdd..7e33b0b 100644
--- a/drivers/firewire/sbp2.c
+++ b/drivers/firewire/sbp2.c
@@ -1014,7 +1014,8 @@
 	return 0;
 }
 
-static int sbp2_scan_logical_unit_dir(struct sbp2_target *tgt, u32 *directory)
+static int sbp2_scan_logical_unit_dir(struct sbp2_target *tgt,
+				      const u32 *directory)
 {
 	struct fw_csr_iterator ci;
 	int key, value;
@@ -1027,7 +1028,7 @@
 	return 0;
 }
 
-static int sbp2_scan_unit_dir(struct sbp2_target *tgt, u32 *directory,
+static int sbp2_scan_unit_dir(struct sbp2_target *tgt, const u32 *directory,
 			      u32 *model, u32 *firmware_revision)
 {
 	struct fw_csr_iterator ci;