Input: tm2-touchkey - use LEN_ON as boolean value instead of LED_FULL

Commit 4e552c8c ("leds: add LED_ON brightness as boolean value")
has introduced the LED_ON enumeration value that can be used
instead of LED_FULL which has more of a linear value.

Because the tm2-touchscreen doesn't have brightness levels, but
it's a simple on/off led, use LED_ON instead of LED_FULL.

Signed-off-by: Andi Shyti <andi.shyti@samsung.com>
Reviewed-by: Jaechul Lee <jcsing.lee@samsung.com>
Tested-by: Jaechul Lee <jcsing.lee@samsung.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>