igc: Remove references to SYSTIMR register

In i225, it's no longer necessary to use the SYSTIMR register to
latch the timer value, the timestamp is latched when SYSTIML is read.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>