[udp] Move high-frequency debug messages to DBGLVL_EXTRA

This makes it possible to leave UDP debugging enabled in order to see
interesting UDP events, without flooding the console with at least one
message per packet.

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

src/net/udp.c

 	udphdr->chksum = tcpip_chksum ( udphdr, len );
 
 	/* Dump debugging information */
-	DBGC ( udp, "UDP %p TX %d->%d len %d\n", udp,
-	       ntohs ( udphdr->src ), ntohs ( udphdr->dest ),
-	       ntohs ( udphdr->len ) );
+	DBGC2 ( udp, "UDP %p TX %d->%d len %d\n", udp,
+		ntohs ( udphdr->src ), ntohs ( udphdr->dest ),
+		ntohs ( udphdr->len ) );
 
 	/* Send it to the next layer for processing */
 	if ( ( rc = tcpip_tx ( iobuf, &udp_protocol, src, dest, netdev,
 	iob_pull ( iobuf, sizeof ( *udphdr ) );
 
 	/* Dump debugging information */
-	DBGC ( udp, "UDP %p RX %d<-%d len %zd\n", udp,
-	       ntohs ( udphdr->dest ), ntohs ( udphdr->src ), ulen );
+	DBGC2 ( udp, "UDP %p RX %d<-%d len %zd\n", udp,
+		ntohs ( udphdr->dest ), ntohs ( udphdr->src ), ulen );
 
 	/* Ignore if no matching connection found */
 	if ( ! udp ) {