net: sfp: add quirk for 2.5G copper SFP

Add a quirk for a copper SFP that identifies itself as "OEM"
"SFP-2.5G-T". This module's PHY is inaccessible, and can only run
at 2500base-X with the host without negotiation. Add a quirk to
enable the 2500base-X interface mode with 2500base-T support, and
disable autonegotiation.

Reported-by: Frank Wunderlich <frank-w@public-files.de>
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>