igb: SerDes flow control setting

This path allows users to get appropriate flow control setting on SerDes
devices, based on original implementation for Copper devices.
Also, since 100baseFX does not support setting flow control, so exclude
it from the setting mechanism.

Signed-off-by: Akeem G. Abodunrin <akeem.g.abodunrin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>