igc: Remove set but not used variables 'ctrl_ext, link_mode'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/intel/igc/igc_base.c: In function 'igc_init_phy_params_base':
drivers/net/ethernet/intel/igc/igc_base.c:240:6: warning:
 variable 'ctrl_ext' set but not used [-Wunused-but-set-variable]
  u32 ctrl_ext;

drivers/net/ethernet/intel/igc/igc_base.c: In function 'igc_get_invariants_base':
drivers/net/ethernet/intel/igc/igc_base.c:290:6: warning:
 variable 'link_mode' set but not used [-Wunused-but-set-variable]
  u32 link_mode = 0;

It never used since introduction in
commit c0071c7a ("igc: Add HW initialization code")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-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>