[netdevice] Do not force a poll on net_tx()

Including a netdev_poll() within net_tx() can cause the net_step()
loop to end up processing hundreds or thousands of packets within a
single step, since each received packet being processed may trigger a
response which, in turn causes a poll for further received packets.

Network devices must now ensure that the TX ring is at least as large
as the RX ring, in order to avoid running out of TX descriptors.  This
should not cause any problems; unlike the RX ring, there is no
substantial memory cost incurred by increasing the TX ring size.

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

src/net/netdevice.c

 	struct ll_protocol *ll_protocol = netdev->ll_protocol;
 	int rc;
 
-	/* Force a poll on the netdevice to (potentially) clear any
-	 * backed-up TX completions.  This is needed on some network
-	 * devices to avoid excessive losses due to small TX ring
-	 * sizes.
-	 */
-	netdev_poll ( netdev );
-
 	/* Add link-layer header */
 	if ( ( rc = ll_protocol->push ( netdev, iobuf, ll_dest, ll_source,
 					net_protocol->net_proto ) ) != 0 ) {