Fixed incorrect semaphore handling on lwip connect and socket_connect

- Semaphore returns 0 on timeout, and negative was incorrect used for
  errors
- Correctly checked error code on tcp_connect

thanks to @LiyouZhou
fixes ARMmbed#284, fixes ARMmbed#285, fixes ARMmbed#166

Author: geky

LWIPInterface.cpp

@@ -110,7 +110,7 @@ int LWIPInterface::connect()
 
     // Wait for an IP Address
     // -1: error, 0: timeout
-    if (netif_up.wait(2500) < 0) {
+    if (netif_up.wait(2500) <= 0) {
         return NSAPI_ERROR_DHCP_FAILURE;
     }
 
@@ -291,10 +291,10 @@ int LWIPInterface::socket_connect(void *handle, const SocketAddress &addr)
     Semaphore connected(0);
     s->sem = &connected;
 
-    tcp_connect(s->tcp, &ip_addr, addr.get_port(), tcp_connect_irq);
+    err_t err = tcp_connect(s->tcp, &ip_addr, addr.get_port(), tcp_connect_irq);
 
     // Wait for connection
-    if (connected.wait(1500) < 0) {
+    if (err || connected.wait(1500) <= 0) {
         return NSAPI_ERROR_NO_CONNECTION;
     }