HID: wiimote: add "bboard_calib" attribute

Balance-Boards provide 3 16bit calibration values for each of the 4
sensors. We provide these now as 192bit value via a new "bboard_calib"
sysfs attribute.
We also re-read the calibration data from the device whenever user-space
attempts to read this file. On normal Nintendo boards, this always
produces the same results, however, on some 3rd party devices these values
change until the device is fully initialized. As I have currently no idea
how long to wait until it's ready (sometimes takes up to 10s?) we provide
a simple workaround for users by reading this file.

If we, at some point, figure out how it works, we can implement it in the
kernel and provide offline data via "bboard_calib". This won't break
user-space then.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>