i40e: Record number TXes cleaned during NAPI

Update i40e_clean_tx_irq to take an out parameter (tx_cleaned) which stores
the number TXs cleaned.

No XDP related TX code is touched. Care has been taken to avoid changing
the control flow of i40e_clean_tx_irq and i40e_napi_poll.

Signed-off-by: Joe Damato <jdamato@fastly.com>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Acked-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>