netlink: specs: ethtool: Remove UAPI duplication of phy-upstream enum

netlink: specs: ethtool: Remove UAPI duplication of phy-upstream enum

The phy-upstream enum is already defined in the ethtool.h UAPI header
and used by the ethtool userspace tool. However, the ethtool spec does
not reference it, causing YNL to auto-generate a duplicate and redundant
enum.

Fix this by updating the spec to reference the existing UAPI enum
in ethtool.h.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20250425171419.947352-1-kory.maincent@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>