i40e: eliminate division in napi_poll data path

Eliminate a division in the napi_poll data path. This division is
executed even though it is only needed in the rare case when there are
not enough interrupt lines so they have to be shared between queue
pairs. Instead, just test for this case and only execute the division
if needed. The code has been lifted from the ice driver.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>