nfp: move temporary variables in nfp_net_tx_complete()

Move temporary variables in scope of the loop in nfp_net_tx_complete(),
and add a temp for txbuf software structure.  This saves us 0.2% of CPU.

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>