HID: wacom: Report 2nd-gen Intuos Pro S center button status over BT

The state of the center button was not reported to userspace for the
2nd-gen Intuos Pro S when used over Bluetooth due to the pad handling
code not being updated to support its reduced number of buttons. This
patch uses the actual number of buttons present on the tablet to
assemble a button state bitmap.

Link: https://github.com/linuxwacom/xf86-input-wacom/issues/112


Fixes: cd47de45b855 ("HID: wacom: Add 2nd gen Intuos Pro Small support")
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Cc: stable@vger.kernel.org # v5.3+
Signed-off-by: Jiri Kosina <jkosina@suse.cz>