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".

contrib/examples/example_app/test.c

@@ -258,11 +258,11 @@ pppLinkStatusCallback(ppp_pcb *pcb, int errCode, void *ctx)
 
 #if PPPOS_SUPPORT
 static u32_t
-ppp_output_cb(ppp_pcb *pcb, u8_t *data, u32_t len, void *ctx)
+ppp_output_cb(ppp_pcb *pcb, const void *data, u32_t len, void *ctx)
 {
   LWIP_UNUSED_ARG(pcb);
   LWIP_UNUSED_ARG(ctx);
-  return sio_write(ppp_sio, data, len);
+  return sio_write(ppp_sio, (u8_t*)data, len);
 }
 #endif /* PPPOS_SUPPORT */
 #endif /* USE_PPP */

contrib/examples/ppp/pppos_example.c

@@ -156,11 +156,11 @@ ppp_link_status_cb(ppp_pcb *pcb, int err_code, void *ctx)
 }
 
 static u32_t
-ppp_output_cb(ppp_pcb *pcb, u8_t *data, u32_t len, void *ctx)
+ppp_output_cb(ppp_pcb *pcb, const void *data, u32_t len, void *ctx)
 {
   LWIP_UNUSED_ARG(pcb);
   LWIP_UNUSED_ARG(ctx);
-  return sio_write(ppp_sio, data, len);
+  return sio_write(ppp_sio, (u8_t*)data, len);
 }
 
 #if LWIP_NETIF_STATUS_CALLBACK

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);
 }
 

src/include/netif/ppp/pppos.h

@@ -58,7 +58,7 @@ enum {
 };
 
 /* PPPoS serial output callback function prototype */
-typedef u32_t (*pppos_output_cb_fn)(ppp_pcb *pcb, u8_t *data, u32_t len, void *ctx);
+typedef u32_t (*pppos_output_cb_fn)(ppp_pcb *pcb, const void *data, u32_t len, void *ctx);
 
 /*
  * Extended asyncmap - allows any character to be escaped.