firewire: core: add CSR MAINT_UTILITY support

Implement the MAIN_UTILITY register, which is utterly optional
but useful as a safe target for diagnostic read/write/broadcast
transactions.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
diff --git a/include/linux/firewire.h b/include/linux/firewire.h
index a50377d..f1160e8 100644
--- a/include/linux/firewire.h
+++ b/include/linux/firewire.h
@@ -38,6 +38,7 @@
 #define CSR_CHANNELS_AVAILABLE		0x224
 #define CSR_CHANNELS_AVAILABLE_HI	0x224
 #define CSR_CHANNELS_AVAILABLE_LO	0x228
+#define CSR_MAINT_UTILITY		0x230
 #define CSR_BROADCAST_CHANNEL		0x234
 #define CSR_CONFIG_ROM			0x400
 #define CSR_CONFIG_ROM_END		0x800
@@ -122,6 +123,8 @@
 	bool broadcast_channel_allocated;
 	u32 broadcast_channel;
 	__be32 topology_map[(CSR_TOPOLOGY_MAP_END - CSR_TOPOLOGY_MAP) / 4];
+
+	__be32 maint_utility_register;
 };
 
 struct fw_attribute_group {