Davinci: Do not reset EMAC TX overruns counter on read

Don't reset tx_fifo_errors when reading out current EMAC stats.
  (tx_fifo_errors shows up as TX overruns in netdev stats.)

Without this correction, the old counter value is lost every time
stats are read out.

Signed-off-by: Thomas Lange <thomas@corelatus.se>
Signed-off-by: David S. Miller <davem@davemloft.net>