Now that the driver core can properly handle constant struct bus_type,
move the hid_bus_type variable to be a constant structure as well,
placing it into read-only memory which can not be modified at runtime.

Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: linux-input@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jiri Kosina <jkosina@suse.com>