Input: xpad - move the input device creation to a new function

To allow us to later create / destroy the input device from the urb
callback, we need to initialize/ deinitialize the input device from a
separate function.  So pull that logic out now to make later patches
more "obvious" as to what they do.

Signed-off-by: "Pierre-Loup A. Griffais" <pgriffais@valvesoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Pavel Rojtberg <rojtberg@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>