[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>

src/drivers/net/intel.c

 static int intel_create_ring ( struct intel_nic *intel,
 			       struct intel_ring *ring ) {
 	physaddr_t address;
+	uint32_t dctl;
 
 	/* Allocate descriptor ring.  Align ring on its own size to
 	 * prevent any possible page-crossing errors due to hardware
 	writel ( 0, ( intel->regs + ring->reg + INTEL_xDH ) );
 	writel ( 0, ( intel->regs + ring->reg + INTEL_xDT ) );
 
+	/* Enable ring */
+	dctl = readl ( intel->regs + ring->reg + INTEL_xDCTL );
+	dctl |= INTEL_xDCTL_ENABLE;
+	writel ( dctl, intel->regs + ring->reg + INTEL_xDCTL );
+
 	DBGC ( intel, "INTEL %p ring %05x is at [%08llx,%08llx)\n",
 	       intel, ring->reg, ( ( unsigned long long ) address ),
 	       ( ( unsigned long long ) address + ring->len ) );

src/drivers/net/intel.h

 /** Receive/Transmit Descriptor Tail (offset) */
 #define INTEL_xDT 0x18
 
+/** Receive/Transmit Descriptor Control (offset) */
+#define INTEL_xDCTL 0x28
+#define INTEL_xDCTL_ENABLE	0x02000000UL	/**< Queue enable */
+
 /** Receive Descriptor Head */
 #define INTEL_RDH ( INTEL_RD + INTEL_xDH )