i40e: free skb after clearing lock in ptp_stop

Use the same logic to free the skb after clearing the Tx timestamp bit
lock in i40e_ptp_stop as we use in the other locations. It is not as
important here since we are not racing against a future Tx timestamp
request (as we are disabling PTP at this point). However it is good to
be consistent in how we approach the bit lock so that future callers
don't copy the old anti-pattern.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>