image: Fix FIT image loadable section custom processing

The original commit always generated linker list entries with the same
entry variable name, because _function passed to ll_entry_declare() is
not a variable and therefore was interpreted as fixed string. Change it
to _type which is a variable and which does allow generation of multiple
unique linker list entries, one for each U_BOOT_FIT_LOADABLE_HANDLER().

Fixes: d7be50921ed3 ("image: Add FIT image loadable section custom processing")
Signed-off-by: Marek Vasut <marek.vasut@mailbox.org>
diff --git a/include/image.h b/include/image.h
index 4620782..1e1bded 100644
--- a/include/image.h
+++ b/include/image.h
@@ -2134,7 +2134,7 @@
  * _handler is the handler function to call after this image type is loaded
  */
 #define U_BOOT_FIT_LOADABLE_HANDLER(_type, _handler) \
-	ll_entry_declare(struct fit_loadable_tbl, _function, fit_loadable) = { \
+	ll_entry_declare(struct fit_loadable_tbl, _type, fit_loadable) = { \
 		.type = _type, \
 		.handler = _handler, \
 	}