net: ethtool: handle EOPNOTSUPP from ethtool get_ts_info() method

net: ethtool: handle EOPNOTSUPP from ethtool get_ts_info() method

Network drivers sometimes return -EOPNOTSUPP from their get_ts_info()
method, and this should not cause the reporting of PHY timestamping
information to be prohibited. Handle this error code, and also
arrange for ethtool_net_get_ts_info_by_phc() to return -EOPNOTSUPP
when the method is not implemented.

This allows e.g. PHYs connected to DSA switches which support
timestamping to report their timestamping capabilities.

Fixes: b9e3f7dc9ed9 ("net: ethtool: tsinfo: Enhance tsinfo to support several hwtstamp by net topology")
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/E1uwiW3-00000004jRF-3CnC@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>