i40e: return the number of enabled queues for ETHTOOL_GRXRINGS

This patch fixes a problem where using ethtool rxnfc command could
let RX flow hash be set on disabled queues. This patch fixes the
problem by returning the number of enabled queues before setting
rxnfc.

Change-ID: Idbac86b0b47ddacc8deee7cd257e41de01cbe5c0
Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>