igb: add function to get maximum RSS queues

This patch adds a new function igb_get_max_rss_queues() to get maximum
RSS queues, this will reduce duplicate code and facilitate future
maintenance.

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>