i40e: remove duplicate stat calculation for tx_errors

The tx_errors statistic was being calculated twice in
i40e_update_eth_stats.

This appears to be as of commit 201db2898f2c ("i40e: add missing VSI
statistics", 2014-03-25).

Remove the extra i40e_stat_update32 call for GLV_TEPC.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>