igc: Remove duplicate code in Tx timestamp handling

The functions igc_ptp_tx_hang() and igc_ptp_tx_work() have duplicate
code which handles Tx timestamp timeouts. This patch does a trivial
refactoring by moving that code to its own function and reusing it.

Signed-off-by: Andre Guedes <andre.guedes@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>