Removed warning when assigning from u32_t to u16_t by casting to u16_t.

src/core/tcp.c

@@ -575,7 +575,7 @@ tcp_slowtmr(void)
 
         /* Reduce congestion window and ssthresh. */
         eff_wnd = LWIP_MIN(pcb->cwnd, pcb->snd_wnd);
-        pcb->ssthresh = eff_wnd >> 1;
+        pcb->ssthresh = (u16_t)(eff_wnd >> 1);
         if (pcb->ssthresh < pcb->mss) {
           pcb->ssthresh = pcb->mss * 2;
         }