igb: Add ethtool support to configure number of channels

This patch adds the ethtool callbacks necessary to configure the
number of RSS queues.

The maximum number of queues is in accordance with the datasheets.

Signed-off-by: Laura Mihaela Vasilescu <laura.vasilescu@rosedu.org>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>