[intel] Explicitly enable descriptor queues

[intel] Explicitly enable descriptor queues

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>