HID: logitech-hidpp: Force feedback support for the Logitech G920

This patch implements force feedback support for the Logitech
G920 Driving Force Racing Wheel. It is a generic implementation
of feature 0x8123 of the Logitech HID++ protocol and should be
usable for any future devices that implement this feature.

This patch should be applied after the basic G920 support patch
by Simon Wood:
http://www.spinics.net/lists/linux-input/msg42174.html

The driving supports everything that is supported by the G920
firmware:
    FF_CONSTANT
    FF_PERIODIC
    FF_SINE
    FF_SQUARE
    FF_SAW_UP
    FF_SAW_DOWN
    FF_TRIANGLE
    FF_SPRING
    FF_DAMPER
    FF_AUTOCENTER
    FF_GAIN
and for version 2 firmware also:
    FF_FRICTION
    FF_INERTIA
    FF_RAMP

Both envelopes and replay values are supported as well, but some
problems may occur when using firmware release 1. There is also a
small residual clockwise damper in the wheel when using the first
firmware release. All problems are fixed in the soon te be released
firmware version 2.

The default spring is disabled by permanently placing a spring
force in the wheel. This spring is also used as the autocenter
spring.

Note: The wheel _DOES_NOT_ auto switch to Logitech/HID mode (it is stuck
in XBox since the xpad changes where not included). Michal has an
alternative approach documented here (and the changes should be submitted
upstream to usb_modeswitch project):

===
Create a file named "046d:c261" in "/etc/usb_modeswitch.d" with the
following content:

DefaultVendor=046d
DefaultProduct=c261
MessageEndpoint=01
ResponseEndpoint=01
TargetClass=0x03
MessageContent="0f00010142"

Then run "usb_modeswitch -c /etc/modeswitch.d/046d:c291" as root and
watch the magic happen:)
===

[jkosina@suse.cz: added information about mode switching from Simon]
[jkosina@suse.cz: fixed a few stylistic issues pointed out by Simon]
[jkosina@suse.cz: fix merge conflict due to to_hid_device() changes]
Signed-off-by: Edwin Velds <e.velds@gmail.com>
Tested-by: Elias Vanderstuyft <elias.vds@gmail.com>
Tested-by: Simon Wood <simon@mungewell.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>