nfp: update ethtool reporting of pauseframe control

Pauseframe control is set to symmetric mode by default on the NFP.
Pause frames can not be configured through ethtool now, but ethtool can
report the supported mode.

Fixes: 265aeb51 ("nfp: add support for .get_link_ksettings()")
Signed-off-by: Fei Qin <fei.qin@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>