r8169: add support for Byte Queue Limits

This patch is basically a resubmit of 1e918876 ("r8169: add support
for Byte Queue Limits") which was reverted later. The problems causing
the revert seem to have been fixed in the meantime.
Only change to the original patch is that the call to
netdev_reset_queue was moved to rtl8169_tx_clear.

The Tested-by refers to a system using the RTL8168evl chip version.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: David S. Miller <davem@davemloft.net>