octeontx2-af: fix 50G ethtool mapping

commit bb000f42a8778607c5355bb611275d08ec8b92a7 from
git@git.assembla.com:cavium/WindRiver.linux.git

OcteonTx2 supports 2lane 50G serdes mode and CN10K supports
single lane 50G serdes mode.

Fix serdes mode to ethtool link mode mapping for OceonTx2 50G links.
They are incorrect mapped to single lane ethtool modes.

Change-Id: Ieb1ecf84ebed218fb012755110729403dfbe8828
Signed-off-by: Hariprasad Kelam <hkelam@marvell.com>
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/56975


Reviewed-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Tested-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>