HID: wiimote: Add sysfs support to wiimote driver

Add sysfs files for each led of the wiimote. Writing 1 to the file
enables the led and 0 disables the led.

We do not need memory barriers when checking wdata->ready since we use
a spinlock directly after it.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>