igc: Remove the 'igc_read_mac_addr_base' method

Remove the redundant 'igc_read_mac_addr_base' method and use
the 'igc_read_mac_addr' method directly instead.

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>