igc: Cleanup the redundant code

The default flow control settings for the i225 device is both
'rx' and 'tx' pause frames. There is no depend on the NVM value.
This patch comes to fix this and clean up the driver code.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>