e1000e: remove redundant if clause from PTP work

tx_hwtstamp_skb is always set before work is scheduled,
work is cancelled before tx_hwtstamp_skb is set to NULL.
PTP work cannot ever see tx_hwtstamp_skb set to NULL.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>