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 a61474c4 ("nfp: ethtool: support reporting link modes").

Fixes: 7c698737

 ("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>