igb: Streamline RSS queue and queue pairing assignment logic.

Rather than spread out the complexity of the RSS queue and queue pairing
assignment logic, place it all in one location for simplicity and
readability.

Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>