[3c5x9] Avoid use of sleep() in driver code

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

src/drivers/net/3c5x9.c

 	else if (connector == utp) {
 		GO_WINDOW(nic->ioaddr,4);
 		outw(ENABLE_UTP, nic->ioaddr + EP_W4_MEDIA_TYPE);
-		sleep(2);	/* Give time for media to negotiate */
+		mdelay(2000);	/* Give time for media to negotiate */
 		GO_WINDOW(nic->ioaddr,1);
 	}