net: phy: adin: allow control of Fast Link Down

Add support to allow Fast Link Down (aka "Enhanced link detection") to
be controlled via the ETHTOOL_PHY_FAST_LINK_DOWN tunable.  These PHYs
have this feature enabled by default.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Acked-by: Nuno Sa <nuno.sa@analog.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20231127-adin-fld-v1-1-797f6423fd48@axis.com


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