net: calxedaxgmac: remove some unused statistic counters

rx_sa_filter_fail and tx_undeflow events are unused and impossible
to occur based on how the h/w is used. We never filter on source MAC
address and TX store and forward mode prevents underflow events.

Reported-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: David S. Miller <davem@davemloft.net>