chore: fix `clang-tidy` warnings

Fix warnings introduced by previous commit.

Change-Id: I3884f8a774860905ddf3d4554f986fde16ec9919
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/inc/hf/sp_pkg.h b/inc/hf/sp_pkg.h
index 182840a..3eb2b90 100644
--- a/inc/hf/sp_pkg.h
+++ b/inc/hf/sp_pkg.h
@@ -39,7 +39,7 @@
 static inline size_t sp_pkg_get_mem_size(struct sp_pkg_header *sp_pkg)
 {
 	assert(SIZE_MAX - sp_pkg->img_offset >= (size_t)sp_pkg->img_size);
-	return (size_t)(sp_pkg->img_offset + sp_pkg->img_size);
+	return (sp_pkg->img_offset + sp_pkg->img_size);
 }
 
 /** Get the size of the boot information descriptors section. */