Fix nanoKONTROL2 set-external-led-mode! so there is no crash when (connect) is called

Before this fix, the `connect` routine called `set-external-led-mode!` with the `out` parameter.  But, the `set-external-led-mode!` code tries to dereference that via `(:out nko)` which returns nil and causes troubles when that is used in the `midi-sysex` call.   

This patch changes `set-external-led-mode!` to expect an `out` input and pass that to `midi-sysex`.  With this fix, the call to `(connect)` now works for me.