arm: Kirkwood: Check the error summary bit for error detection

The Marvell documentation for the 88f6281 states that the error coding
is only valid if the error summary and last frame bits in the transmit
descriptor status field are set. This patch adds checks for these for
transmit (I would get transmit errors on bootp with the current check,
which I believe are spurious).

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>