leds: Provide stubs for when CLASS_LED & NEW_LEDS are disabled

Provide stubs for devm_led_classdev_register_ext() and
led_init_default_state_get() so that LED drivers embedded within other
drivers such as PHYs and Ethernet switches still build when LEDS_CLASS
or NEW_LEDS are disabled. This also helps with Kconfig dependencies,
which are somewhat hairy for phylib and mdio and only get worse when
adding a dependency on LED_CLASS.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>