net: phy: Don't register LEDs for genphy

If a PHY has no driver, the genphy driver is probed/removed directly in
phy_attach/detach. If the PHY's ofnode has an "leds" subnode, then the
LEDs will be (un)registered when probing/removing the genphy driver.
This could occur if the leds are for a non-generic driver that isn't
loaded for whatever reason. Synchronously removing the PHY device in
phy_detach leads to the following deadlock:

rtnl_lock()
ndo_close()
...
phy_detach()
phy_remove()
phy_leds_unregister()
led_classdev_unregister()
led_trigger_set()
netdev_trigger_deactivate()
unregister_netdevice_notifier()
rtnl_lock()

There is a corresponding deadlock on the open/register side of things
(and that one is reported by lockdep), but it requires a race while this
one is deterministic. Regular drivers do not have this problem since
they are probed asynchronously (without RTNL held).

Generic PHYs do not support LEDs anyway, so don't bother registering
them.

[JakubL this is a net-next version of
commit f0f2b992d818 ("net: phy: Don't register LEDs for genphy"),
which uses APIs removed in -next.]

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Link: https://patch.msgid.link/20250710201454.1280277-1-sean.anderson@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>