params: make param_t uint16_t on NuttX

param_t is only used as an offset and we have <1000 params, so an uint16_t
is enough.
This saves roughly 1KB of RAM. We only do that on NuttX because normal
integers have better performance in general.
Previously on amd64, this was even 64bits because it was an uintptr_t.

src/modules/systemlib/flashparams/flashparams.c

@@ -65,8 +65,8 @@
  * Storage for modified parameters.
  */
 struct param_wbuf_s {
-	param_t                 param;
 	union param_value_u     val;
+	param_t                 param;
 	bool                    unsaved;
 };