net: phy: fix circular LEDS_CLASS dependencies

The CONFIG_PHYLIB symbol is selected by a number of device drivers that
need PHY support, but it now has a dependency on CONFIG_LEDS_CLASS,
which may not be enabled, causing build failures.

Avoid the risk of missing and circular dependencies by guarding the
phylib LED support itself in another Kconfig symbol that can only be
enabled if the dependency is met.

This could be made a hidden symbol and always enabled when both CONFIG_OF
and CONFIG_LEDS_CLASS are reachable from the phylib, but there may be an
advantage in having users see this option when they have a misconfigured
kernel without built-in LED support.

Fixes: 01e5b728

 ("net: phy: Add a binding for PHY LEDs")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20230420084624.3005701-1-arnd@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>