Set tool device ID for generic protocol hardware

The code in usbParseKeyEvent which is supposed to set a device
ID ignores the case of devices which use the generic protocol.
This patch changes the logic so that a device ID is assigned
from the BTN_* event for any non-protocol 5 device.

Fixes: https://sourceforge.net/p/linuxwacom/bugs/246/
Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Reviewed-by: Ping Cheng <pingc@wacom.com>