fix(perf): Increase threshold for jumbo

fix(perf): Increase threshold for jumbo

The old limit of 1522 was introduced long time ago.
First, it appeared here [0], where it is correct
as that suite has zero overhead.
The first suite with wrong logic seems to be here [1]
(no "Add No Multi Seg to all DUTs" in 1518B testcase).
And when I was moving that logic to a keyword in [2],
I did not realize it is wrong with overhead.

This Change uses 1800 as the new threshold,
matching the value used for non-jumbo MTU.

[0] https://gerrit.fd.io/r/c/csit/+/2652/12/tests/perf/Bridge_Domain_Intel-X520-DA2.robot#70
[1] https://gerrit.fd.io/r/c/csit/+/4454/96/tests/perf/40ge2p1xl710-ethip4ipsecscaleip4-ip4base-interfaces-aes-gcm-ndrpdrdisc.robot#229
[2] https://gerrit.fd.io/r/c/csit/+/13411/35/resources/libraries/robot/performance/performance_utils.robot#84

Change-Id: Iff3703fcff0e4bbb1a6b10be359fa5ef67fd5422
Signed-off-by: Vratko Polak <vrpolak@cisco.com>