net: ena: limit refill Rx threshold to 256 to avoid latency issues

net: ena: limit refill Rx threshold to 256 to avoid latency issues

BugLink: http://bugs.launchpad.net/bugs/1798182
Currently Rx refill is done when the number of required descriptors is
above 1/8 queue size. With a default of 1024 entries per queue the
threshold is 128 descriptors.
There is intention to increase the queue size to 8196 entries.
In this case threshold of 1024 descriptors is too large and can hurt
latency.
Add another limitation to Rx threshold to be at most 256 descriptors.

Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 0574bb806dad29a3dada0ee42b01645477d48282 linux-next)
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Acked-by: Khalid Elmously <khalid.elmously@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>