HID: wacom: fix bit shift for Cintiq Companion 2

The bit indicating BTN_6 on this device is overshifted
by 2 bits, resulting in the incorrect button being
reported.

Also fix copy-paste mistake in comments.

Signed-off-by: Aaron Armstrong Skomra <aaron.skomra@wacom.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Link: https://github.com/linuxwacom/xf86-input-wacom/issues/71
Fixes: c7f0522a

 ("HID: wacom: Slim down wacom_intuos_pad processing")
Cc: <stable@vger.kernel.org> # v4.5+
Signed-off-by: Jiri Kosina <jkosina@suse.cz>