bnxt_en: Copy and paste bug in extended tx_stats

The struct type was copied from the line before but it should be "tx"
instead of "rx".  I have reviewed the code and I can't immediately see
that this bug causes a runtime issue.

Fixes: 36e53349

 ("bnxt_en: Add additional extended port statistics.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>