HID: google: whiskers: signal tablet mode on connect

When we receive "keyboard position" event from Whiskers we can
conclude that the base is attached, even if we did not get EC event
for that. We may miss EC event because there are some units which
have a lot of leakage on the ADC pins that the EC uses to determine
whether or not a base is attached.

Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>