HID: wacom: Properly report negative values from Intuos Pro 2 Bluetooth

The wacom driver's IRQ handler for Bluetooth reports from the 2nd-gen
Intuos Pro does not correctly process negative numbers. Values for
tilt and rotation (which can go negative) are instead interpreted as
unsigned and so jump to very large values when the data should be
negative. This commit properly casts the data to ensure we report
negative numbers when necessary.

Fixes: 4922cd26 ("HID: wacom: Support 2nd-gen Intuos Pro's Bluetooth classic interface")
Cc: stable@vger.kernel.org # v4.11
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>