[tg3] Add missing memory barrier

ARM64 has a weaker memory order model than x86.  The missing memory
barrier caused phy initialization notification to be delayed beyond
the link-wait timeout (15 secs).

Signed-off-by: Leendert van Doorn <leendert@paramecium.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/tg3/tg3.c

 	 */
 	tp->hw_status->status &= ~SD_STATUS_UPDATED;
 
+	mb();
+
 	tg3_poll_link(tp);
 	tg3_tx_complete(dev);
 	tg3_rx_complete(dev);