nfp: improve link modes reading process

Avoid reading link modes from management firmware every time when
`ethtool_get_link_ksettings` is called, only communicate with
management firmware when necessary like we do for eth_table info.

This change can ease the situation that when large number of vlan
sub-interfaces are created and their information is requested by
some monitoring process like PCP [1] through ethool ioctl frequently.

[1] https://pcp.io

Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Acked-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20230509075817.10566-1-louis.peens@corigine.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>