HID: microsoft: Convert private data to be a proper struct

In order to be able to have more than just an unsigned long worth of
private data, convert the code to allocate and use a dedicated struct.

Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: linux-input@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Pierre-Loup A. Griffais <pgriffais@valvesoftware.com>
Signed-off-by: Juha Kuikka <juha.kuikka@gmail.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>