[tcp] Use correct length for memset()

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp.c

 		wsopt->wsopt.length = sizeof ( wsopt->wsopt );
 		wsopt->wsopt.scale = TCP_RX_WINDOW_SCALE;
 		spopt = iob_push ( iobuf, sizeof ( *spopt ) );
-		memset ( spopt->nop, TCP_OPTION_NOP, sizeof ( spopt ) );
+		memset ( spopt->nop, TCP_OPTION_NOP, sizeof ( spopt->nop ) );
 		spopt->spopt.kind = TCP_OPTION_SACK_PERMITTED;
 		spopt->spopt.length = sizeof ( spopt->spopt );
 	}