gianfar: Fix reported number of sent bytes to BQL

This is a backported patch for the gianfar ethernet driver
used in TPLink 4900 v1. It is supposed to fix the error which
show up in dmesg with:
NETDEV WATCHDOG: eth0 (fsl-gianfar): transmit queue xy timed out
Full upstream patch is at: http://patchwork.ozlabs.org/patch/271242

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@39692 3c298f89-4303-0410-b956-a3cf2f4a3e73