ixgbe: Cleanup feature setup code to make the code more readable

This is purely a cleanup patch.  This collapses some of the code required
when we configure our Tx and Rx feature sets, and makes the code more
readable and maintainable.

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