HID: asus: Ignore Asus vendor-page usage-code 0xff events

At least on a T100HA an Asus vendor-page usage-code 0xff event is send on
every suspend and again on resume, resulting in the following warning:

asus 0003:0B05:1807.0002: Unmapped Asus vendor usagepage code 0xff

being logged twice on every suspend/resume.

This commit silences the "Unmapped Asus vendor usagepage code ..."
warning for usage-code 0xff to avoid these warnings being logged.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>