bsg: fix initialization error handling bugs

This fixes the following bugs and cleans up the initialization code:

- cdev_del is missing.
- unregister_chrdev_region should be used instead of unregister_chrdev.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>