r8169: fix accessing unset transport header

r8169: fix accessing unset transport header

f588aa3f3718 ("net: warn if transport header was not set") added
a check that triggers a warning in r8169, see [0].

The commit referenced in the Fixes tag refers to the change from
which the patch applies cleanly, there's nothing wrong with this
commit. It seems the actual issue (not bug, because the warning
is harmless here) was introduced with e50413acc35f
("r8169: use Giant Send").

[0] https://bugzilla.kernel.org/show_bug.cgi?id=216157

Fixes: 84567637eba7 ("r8169: work around RTL8125 UDP hw bug")
Reported-by: Erhard F. <erhard_f@mailbox.org>
Tested-by: Erhard F. <erhard_f@mailbox.org>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/1b2c2b29-3dc0-f7b6-5694-97ec526d51a0@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>