net: phylink: disable autoneg for interfaces that have no inband

Mathew reports that as a result of commit 6561f0e5 ("net: pcs:
pcs-lynx: implement pcs_inband_caps() method"), 10G SFP modules no
longer work with the Lynx PCS.

This problem is not specific to the Lynx PCS, but is caused by commit
df874f9e ("net: phylink: add pcs_inband_caps() method") which added
validation of the autoneg state to the optical SFP configuration path.

Fix this by handling interface modes that fundamentally have no
inband negotiation more correctly - if we only have a single interface
mode, clear the Autoneg support bit and the advertising mask. If the
module can operate with several different interface modes, autoneg may
be supported for other modes, so leave the support mask alone and just
clear the Autoneg bit in the advertising mask.

This restores 10G optical module functionality with PCS that supply
their inband support, and makes ethtool output look sane.

Reported-by: Mathew McBride <matt@traverse.com.au>
Closes: https://lore.kernel.org/r/025c0ebe-5537-4fa3-b05a-8b835e5ad317@app.fastmail.com


Fixes: df874f9e ("net: phylink: add pcs_inband_caps() method")
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/E1uslwx-00000001SPB-2kiM@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>