igc: Fix code redundancy

Remove redundant igc_check_for_link_base code and replace it with
an igc_check_for_copper_link method.
Fix duplication of IGC_ADVTXD_PAYLEN_SHIFT mask declaration.
Remove obsolete IGC_SCVPC register definition.

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>