[intel] Refill receive ring only after enabling receiver

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>

src/drivers/net/intel.c

 	if ( ( rc = intel_create_ring ( intel, &intel->rx ) ) != 0 )
 		goto err_create_rx;
 
-	/* Fill receive ring */
-	intel_refill_rx ( intel );
-
 	/* Program MAC address */
 	memset ( &mac, 0, sizeof ( mac ) );
 	memcpy ( mac.raw, netdev->ll_addr, sizeof ( mac.raw ) );
 		  INTEL_RCTL_BAM | INTEL_RCTL_BSIZE_2048 | INTEL_RCTL_SECRC );
 	writel ( rctl, intel->regs + INTEL_RCTL );
 
+	/* Fill receive ring */
+	intel_refill_rx ( intel );
+
 	/* Update link state */
 	intel_check_link ( netdev );