octeontx2-pf: ethtool: Implement get_fec_stats

This patch registers a callback for get_fec_stats such that
FEC stats can be queried from the below command

"ethtool -I --show-fec eth0"

Signed-off-by: Hariprasad Kelam <hkelam@marvell.com>
Signed-off-by: Sunil Kovvuri Goutham <sgoutham@marvell.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>