r8169: align the IP header when there is no DMA constraint

Align the IP header when the chipset can DMA at any location (plain 0x8169).
Otherwise (0x8136/0x8168) obey the constraint imposed by the hardware.

This patch complements the previous alignment rework done for copybreak.

Original idea from Philip Craig <philipc@snapgear.com>

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Philip Craig <philipc@snapgear.com>
Cc: Mike Isely <isely@pobox.com>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>