Fix endPacket() return value if socket is unbound

[jrowberg]

The endPacket() method returns 0 (failure) only in the case where WiFiSocket.sendto() returns < 0. However, the sendto() method returns exactly 0 in the case where the socket is not in a BOUND state. This allows endPacket() to indicate success after sendto() indicates failure. The simple fix is to change the condition to be <= 0 instead of < 0.