ftgmac100: Simplify rx packets error handling

The fast path has a single unlikely() test for any error bit,
calling into a helper that sets the appropriate statistics.

The various netdev_info aren't particularly interesting. If
we want to differentiate the various length errors later we
can introduce driver specific stats using ethtool.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>