mpc85xx: Fix NAPI poll mechanism in GIANFAR ethernet driver

This patch fixes the NAPI poll mechanism in the GIANFAR ethernet driver, which
was not properly working since Linus Kernel Version 3,8.
Therefore the workaround patch to downgrade the GIANFAR ethernet driver to
Kernelversion v3.8 is obsoete.
This patch was extensivly testes with different network loads and types of
traffic. There is quite a substantial user base that reports proper Ethernet
function with TPlink-4900. This patch is based on the fixes from GINAFAR
maintainer Claudiu Manoli.

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38865 3c298f89-4303-0410-b956-a3cf2f4a3e73