i40e: Add basic support for get/set channels for RSS

Implement the number of receive/transmit queue pair being
changed on the fly by ethtool.

Change-Id: I70df2363f1ca40b63610baa561c5b6b92b81bca7
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Kavindya Deegala <kavindya.s.deegala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>