[tcp] Set PSH flag only on packets containing data

Suggested-by: Yelena Kadach <klenusik@hotmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/tcp.c

 		tsopt->tsopt.tsval = htonl ( currticks() );
 		tsopt->tsopt.tsecr = htonl ( tcp->ts_recent );
 	}
-	if ( ! ( flags & TCP_SYN ) )
+	if ( len != 0 )
 		flags |= TCP_PSH;
 	tcphdr = iob_push ( iobuf, sizeof ( *tcphdr ) );
 	memset ( tcphdr, 0, sizeof ( *tcphdr ) );