PPP, CORE, cleaned ppp_close() function

Calling new_phase() instead of assigning pcb->phase directly.
Removed redundant call to link status callback.

src/include/netif/ppp/ppp.h

@@ -469,12 +469,13 @@ err_t ppp_open(ppp_pcb *pcb, u16_t holdoff);
 /*
  * Initiate the end of a PPP connection.
  * Any outstanding packets in the queues are dropped.
- * Return 0 on success, an error code on failure.
  *
  * Setting nocarrier to 1 close the PPP connection without initiating the
  * shutdown procedure. Always using nocarrier = 0 is still recommended,
  * this is going to take a little longer time if your link is down, but
  * is a safer choice for the PPP state machine.
+ *
+ * Return 0 on success, an error code on failure.
  */
 err_t ppp_close(ppp_pcb *pcb, u8_t nocarrier);