igb: fix hw timestamping

Hardware timestamping for Intel 82580 didn't work in either 2.6.36 or
2.6.37. Comparing it to Intel's igb-2.4.12 I found that the
timecounter_init clock/counter initialization was done too early.

Signed-off-by: Anders Berggren <andfers@halon.se>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>