Bigger combo index (#9318)

* Add change log

* Change combo index from uint8_t to uint16_t

docs/ChangeLog/20200829/PR9318.md

@@ -0,0 +1,11 @@
+### Bigger integer type when looping over combos.
+
+[#9318](https://github.com/qmk/qmk_firmware/pull/9318)
+
+Changes `uint8_t` to `uint16_t` so it is possible have more than 256 combos.
+
+Any fork that uses `process_combo_event` needs to update the function's first argument to `uint16_t`.
+
+| Old function                                                  | New Function                                                   |
+|---------------------------------------------------------------|----------------------------------------------------------------|
+| `void process_combo_event(uint8_t combo_index, bool pressed)` | `void process_combo_event(uint16_t combo_index, bool pressed)` |