r8169: WakeOnLan fix for the 8168

More stuff for http://bugzilla.kernel.org/show_bug.cgi?id=9512

Some 8168 are unable to WoL when receiving is not enabled (plain
old 8169 do not seem to care).

It is not exactly pretty to leave the receiver enabled but we
should now enable DMA late enough for it to be safe. Some late
stage boot failure due to pxe and friends may benefit from the
delayed enabling of bus-mastering as well.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Tested-by: Jaromír Cápík <tavvva@volny.cz>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>