nfp: ethtool: fix the bug of setting unsupported port speed

nfp: ethtool: fix the bug of setting unsupported port speed

Unsupported port speed can be set and cause error. Now fixing it
and return an error if setting unsupported speed.

This fix depends on the following, which was included in v6.2-rc1:
commit a61474c41e8c ("nfp: ethtool: support reporting link modes").

Fixes: 7c698737270f ("nfp: add support for .set_link_ksettings()")
Signed-off-by: Yu Xiao <yu.xiao@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>