igb: re-use igb_ptp_reset in igb_ptp_init

Modify igb_ptp_init to take advantage of igb_ptp_reset, and remove
duplicated work that was occurring in both igb_ptp_reset and
igb_ptp_init.

In total, resetting the TSAUXC register, and resetting the system time
both happen in igb_ptp_reset already. igb_ptp_reset now also takes care
of starting the delayed work item for overflow checks, as well.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>