leds: Fix build for LEDS_CLASS=m on versatile

I got a build error today, since LEDS_VERSATILE can be built-in while LEDS_CLASS
is a module:

drivers/built-in.o: In function `versatile_leds_probe':
:(.text+0x155020): undefined reference to `led_classdev_register'

I suggest we turn this option into 'tristate' so that the dependency
tracking works correctly.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>