Rename SMC first argument

'fid' (short for 'function ID') makes for a more explicit name.

Change-Id: I41d90c39979162142b0377a68f4be90dc31de253
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
diff --git a/include/lib/tftf_lib.h b/include/lib/tftf_lib.h
index 40955d3..58112b4 100644
--- a/include/lib/tftf_lib.h
+++ b/include/lib/tftf_lib.h
@@ -131,7 +131,7 @@
  */
 typedef struct {
 	/* Function identifier. Identifies which function is being invoked. */
-	uint32_t	arg0;
+	uint32_t	fid;
 
 	u_register_t	arg1;
 	u_register_t	arg2;