nfp: tls: count TSO segments separately for the TLS offload

Count the number of successfully submitted TLS segments,
not skbs. This will make it easier to compare the TLS
encryption count against other counters.

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>