net: dp83640: Avoid NULL pointer dereference.

The function, skb_complete_tx_timestamp(), used to allow passing in a
NULL pointer for the time stamps, but that was changed in commit
62bccb8c ("net-timestamp: Make the
clone operation stand-alone from phy timestamping"), and the existing
call sites, all of which are in the dp83640 driver, were fixed up.

Even though the kernel-doc was subsequently updated in commit
7a76a021 ("net-timestamp: Update
skb_complete_tx_timestamp comment"), still a bug fix from Manfred
Rudigier came into the driver using the old semantics.  Probably
Manfred derived that patch from an older kernel version.

This fix should be applied to the stable trees as well.

Fixes: 81e8f2e9 ("net: dp83640: Fix tx timestamp overflow handling.")
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>