The analog sticks of the pro-controller might report slightly off values.
To guarantee a uniform setup, we now calibrate analog-stick values during
pro-controller setup.

Unfortunately, the pro-controller fails during normal EEPROM reads and I
couldn't figure out whether there are any calibration values stored on the
device. Therefore, we now use the first values reported by the device (iff
they are not _way_ off, which would indicate movement) to initialize the
calibration values. To allow users to change this calibration data, we
provide a pro_calib sysfs attribute.

We also change the "flat" values so user-space correctly smoothes our
data. It makes slightly off zero-positions less visible while still
guaranteeing highly precise movement reports. Note that the pro controller
reports zero-positions in a quite huge range (at least: -100 to +100).
Reported-by: Rafael Brune <mail@rbrune.de>
Tested-by: Rafael Brune <mail@rbrune.de>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>