On i350 the datasheet contradicts itself in stating that the default
value of RXDCTL.ENABLE for queue zero is both set (according to the
"Receive Initialization" section) and unset (according to the "Receive
Descriptor Control - RXDCTL" section).  Empirical evidence suggests
that the default value is unset.

Explicitly enable both transmit and receive queues to avoid any
ambiguity.

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

On 82576 (and probably others), the datasheet states that "the tail
register of the queue (RDT[n]) should not be bumped until the queue is
enabled".  There is some confusion over exactly what constitutes
"enabled": the initialisation blurb says that we should "poll the
RXDCTL register until the ENABLE bit is set", while the description
for the RXDCTL register says that the ENABLE bit is set by default
(for queue zero).  Empirical evidence suggests that the ENABLE bit
reads as set immediately after writing to RCTL.EN, and so polling is
not necessary.

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

Tested-by: Robin Smidsrød <robin@smidsrod.no>
Tested-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>