HID: wacom: Correct button numbering 2nd-gen Intuos Pro over Bluetooth

The button numbering of the 2nd-gen Intuos Pro is not consistent between
the USB and Bluetooth interfaces. Over USB, the HID_GENERIC codepath
enumerates the eight ExpressKeys first (BTN_0 - BTN_7) followed by the
center modeswitch button (BTN_8). The Bluetooth codepath, however, has
the center modeswitch button as BTN_0 and the the eight ExpressKeys as
BTN_1 - BTN_8. To ensure userspace button mappings do not change
depending on how the tablet is connected, modify the Bluetooth codepath
to report buttons in the same order as USB.

To ensure the mode switch LED continues to toggle in response to the
mode switch button, the `wacom_is_led_toggled` function also requires
a small update.

Link: https://github.com/linuxwacom/input-wacom/pull/79


Fixes: 4922cd26 ("HID: wacom: Support 2nd-gen Intuos Pro's Bluetooth classic interface")
Cc: <stable@vger.kernel.org> # 4.11+
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Reviewed-by: Aaron Skomra <aaron.skomra@wacom.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>