lwip: Use modulo operator for mbox

[Ryzee119]

Was getting some corruption in the lwip mboxes. This is a bug and the code should be the modulo operator not bitwise AND.

The below code demonstrates the issue

static void my_tcpip_callback1(void *param){
    DbgPrint("I work\n"):
} 

static void my_tcpip_callback2(void *param){
    DbgPrint("I don't work\n"):
}

nxNetInit(NULL);
tcpip_callback(my_tcpip_callback1, NULL); // This works by chance as mbox position is zero.
tcpip_callback(my_tcpip_callback2, NULL); // This doesnt work