HID: separate mode switching function for wacom bluetooth driver

wacom_poke function allows to switch tablet reporting speed. The patch
dosen't add any new functionality, but it's preparation for user-space
speed switching through sysfs.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>
Signed-off-by: Bastien Nocera <hadess@hadess.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>