r8169: remove now unneeded barrier in rtl_tx

Until ae84bc18 ("r8169: don't use bit LastFrag in tx descriptor
after send") we used to access another bit in the descriptor, therefore
it seems the barrier was needed. Since this commit DescOwn is the
only bit we're interested in, so the barrier isn't needed any longer.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>