ax88179_178a: avoid copy of tx tcp packets

ax88179_tx_fixup() has quite complex code trying to push 8 bytes
of control data (len/mss), but fails to do it properly for TCP packets,
incurring an extra copy and point of memory allocation failure.

Lets use the simple and approved way.

dev->needed_headroom being 8, all frames should have 8 bytes of
headroom, so the extra copy should be unlikely anyway.

This patch should improve performance for TCP xmits.

Reported-by: Ming Lei <ming.lei@canonical.com>
Tested-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>