HID: wacom: Do not register input devices until after hid_hw_start

If a input device is opened before hid_hw_start is called, events may
not be received from the hardware. In the case of USB-backed devices,
for example, the hid_hw_start function is responsible for filling in
the URB which is submitted when the input device is opened. If a device
is opened prematurely, polling will never start because the device will
not have been in the correct state to send the URB.

Because the wacom driver registers its input devices before calling
hid_hw_start, there is a window of time where a device can be opened
and end up in an inoperable state. Some ARM-based Chromebooks in particular
reliably trigger this bug.

This commit splits the wacom_register_inputs function into two pieces.
One which is responsible for setting up the allocated inputs (and runs
prior to hid_hw_start so that devices are ready for any input events
they may end up receiving) and another which only registers the devices
(and runs after hid_hw_start to ensure devices can be immediately opened
without issue). Note that the functions to initialize the LEDs and remotes
are also moved after hid_hw_start to maintain their own dependency chains.

Fixes: 7704ac93

 ("HID: wacom: implement generic HID handling for pen generic devices")
Cc: stable@vger.kernel.org # v3.18+
Suggested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Tested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>