We don't need to refill the rx descriptors on every napi
if only a few were handled.  Waiting until we can batch up
a few together will save us a few Rx cycles.

Signed-off-by: Shannon Nelson <snelson@pensando.io>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>