[netdevice] Allow network device to update link state before checking

If the network interface has only just been opened (e.g. by the "dhcp"
command) then we should allow at least one opportunity for the card to
update the link state before testing it, to avoid false positives.

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

src/usr/ifmgmt.c

 	int key;
 	int rc;
 
+	/* Allow link state to be updated */
+	netdev_poll ( netdev );
+
 	if ( netdev_link_ok ( netdev ) )
 		return 0;