[intel] Avoid completely filling the TX descriptor ring

It is unclear from the datasheets whether or not the TX ring can be
completely filled (i.e. whether writing the tail value as equal to the
current head value will cause the ring to be treated as completely
full or completely empty).  It is very plausible that this edge case
could differ in behaviour between real hardware and the many
implementations of an emulated Intel NIC found in various virtual
machines.  Err on the side of caution and always leave at least one
ring entry empty.

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