[intel] Increase receive ring fill level

As of commit d28bb51 ("[tcp] Defer sending ACKs until all received
packets have been processed"), increasing the RX ring size will
increase the number of received packets per transmitted ACK (since
each poll will process up to one complete receive ring).  Under KVM,
this can make a substantial (up to ~200%) difference to the overall
download speed, since transmissions are very expensive.

Increase the ring fill level from four to eight packets: this
increases the download speed by around 50% at a cost of around 8kB of
heap space.  Further speedups are possible by increasing the ring size
further, but it would be preferable to find alternative methods which
do not use noticeable amounts of heap space.

Tested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>