igb: Force flow control off during reset when forcing speed.

During igb_reset(), we initiate a hardware reset which will clear our
flow control settings. For auto-negotiation, we re-negotiate them when
linking up again, but we need to force them off properly for the forced
speed case.

Signed-off-by: Matthew Vick <matthew.vick@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>