nfp: count all failed TX attempts as errors

Currently if we need to modify the head of the skb and allocation
fails we would free the skb and not increment the error counter.
Make sure all errors are counted.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>