bsg: minor bug fixes

This fixes the following minor issues:

- add EXPORT_SYMBOL_GPL for bsg_register_queue and
bsg_unregister_queue.

- shut up gcc warnings

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <axboe@nelson.home.kernel.dk>