PPP, PPPoS: Use const void* instead of u8_t* in pppos_output_cb_fn()

There is no good reason why this function should take a non-const
pointer, as the output callback should never modify what lwIP gives it.
While changing that also switch to a more generic `void*` instead of
"byte".

doc/ppp.txt

@@ -195,7 +195,7 @@ static void status_cb(ppp_pcb *pcb, int err_code, void *ctx) {
  *
  * Return value: len if write succeed
  */
-static u32_t output_cb(ppp_pcb *pcb, u8_t *data, u32_t len, void *ctx) {
+static u32_t output_cb(ppp_pcb *pcb, const void *data, u32_t len, void *ctx) {
   return uart_write(UART, data, len);
 }