net/ena: limit refill threshold by fixed value

net/ena: limit refill threshold by fixed value

Divider used for both Tx and Rx cleanup/refill threshold can cause too
big delay in case of the really big rings - for example if the 8k Rx
ring will be used, the refill won't trigger unless 1024 threshold will
be reached. It will also cause driver to try to allocate that much
descriptors.

Limiting it by fixed value - 256 in that case, would limit maximum
time spent in repopulate function.

Signed-off-by: Michal Krawczyk <mk@semihalf.com>
Reviewed-by: Igor Chauskin <igorch@amazon.com>
Reviewed-by: Guy Tzalik <gtzalik@amazon.com>