qlcnic: Fix update of ethtool stats.

o Aggregating tx stats in adapter variable was resulting in
  an increase in stats even after no traffic was run and
  user runs ifconfig/ethtool command.
o qlcnic_update_stats used to accumulate stats in adapter
  struct at each function call, instead accumulate tx stats
  in local variable and then assign it to adapter structure.

Reported-by: Holger Kiehl <holger.kiehl@dwd.de>
Signed-off-by: Rajesh Borundia <rajesh.borundia@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>