[smsc75xx] Move RX FIFO overflow message to DBGLVL_EXTRA

RX FIFO overflow is almost inevitable since the (usable) USB2 bus
bandwidth is approximately one quarter of the Ethernet bandwidth.
Avoid flooding the console with RX FIFO overflow messages in a
standard debug build.

With TCP SACK implemented, the RX FIFO overflow no longer causes a
catastrophic drop in throughput.  Experimentation shows that HTTP
downloads now progress at a fairly smooth 250Mbps, which is around the
maximum speed attainable for a USB2 NIC.

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

src/drivers/net/smsc75xx.c

 
 	/* Record RX FIFO overflow if applicable */
 	if ( int_sts & SMSC75XX_INT_STS_RDFO_INT ) {
-		DBGC ( smsc75xx, "SMSC75XX %p RX FIFO overflowed\n", smsc75xx );
+		DBGC2 ( smsc75xx, "SMSC75XX %p RX FIFO overflowed\n",
+			smsc75xx );
 		netdev_rx_err ( netdev, NULL, -ENOBUFS );
 		int_sts &= ~SMSC75XX_INT_STS_RDFO_INT;
 	}