ixgbe: change default ring size

decrease the memory utilization of the tx / rx queue allocation
by changing the default ring size to 512 (from 1024).  At
1024 rx entries of 2KB each (from 4kB slab) with 16 queues
ixgbe was using 64 MB of memory per port, which is not
necessary.

Users can still change queue lengths with ethtool -k.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>