ARM: Orion: Set eth packet size csum offload limit

The mv643xx ethernet controller limits the packet size for the TX
checksum offloading. This patch sets this limits for Kirkwood and
Dove which have smaller limits that the default.

As a side note, this patch is an updated version of a patch sent some years
ago: http://lists.infradead.org/pipermail/linux-arm-kernel/2010-June/017320.html
which seems to have been lost.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>
Cc: <stable@vger.kernel.org>