HID: asus: do not try to initialize the backlight if the enpoint doesn't support it

HID: asus: do not try to initialize the backlight if the enpoint doesn't support it

Avoid possibly printing a warning about the inability to initialize the
backlight if the hid endpoint doesn't support it.

Also fix and move an incorrect check in asus_kbd_register_leds(): that
same check is now used as a precondition to call that function.

Fixes: 0919db9f3583 ("HID: asus: always fully initialize devices")
Signed-off-by: Denis Benato <denis.benato@linux.dev>
Signed-off-by: Jiri Kosina <jkosina@suse.com>