efi_loader: error handling for `efidebug boot add`

In  `efidebug boot add iPXE scsi 0:1 snp-arm64.efi --foo` a parameter is
missing. Hence the command should not silently return as if everything were
ok but should display the usage info.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Alexander Graf <agraf@csgraf.de>
diff --git a/cmd/efidebug.c b/cmd/efidebug.c
index 5072a7b..db96682 100644
--- a/cmd/efidebug.c
+++ b/cmd/efidebug.c
@@ -491,7 +491,7 @@
 
 	id = (int)simple_strtoul(argv[1], &endp, 16);
 	if (*endp != '\0' || id > 0xffff)
-		return CMD_RET_FAILURE;
+		return CMD_RET_USAGE;
 
 	sprintf(var_name, "Boot%04X", id);
 	p = var_name16;